Information recording medium and its manufacturing method

ABSTRACT

A phase change recording medium comprising an as-deposited first recording layer configured to undergo a reversible phase change between an amorphous state and a crystalline state due to light irradiation and thereby change an optical characteristic. The as-deposited first recording layer includes a plurality of fine nuclei having an average size of 0.5 nm to 4 nm in the amorphous state.

BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to an informationrecording medium and its manufacturing method. More specifically, theinvention relates to a phase change recording medium, which is anoptical recording medium having a phase change type optical recordinglayer irradiated with light beams for recording and/or reproducinginformation and which does not need any initial crystallizing steps forcrystallizing the recording layer after the recording layer isdeposited.

[0002] In a phase change type optical recording medium irradiated withoptical beams for recording and/or reproducing information, there areadvantages in that the medium has a large capacity, high-speedaccessibility and medium portability, and that it is possible to moreinexpensively provide a CD interchangeable drive than competitivemagneto-optical media since its reproduction principle is reflectancechange type which is the same as those of CDs. In addition, there areadvantages in that it is possible to easily increase the density of themedium since the medium has an excellent signal quality, and that themedium has a high recorded-data transfer rate.

[0003] The phase change recording medium is able to record informationby forming a recording mark and to erase information by erasing therecording mark. The recording mark is formed by allowing a recordinglayer to be irradiated with a light beam of a recording level to bemolten to be in a random state, and then, by cooling the recording layerin a shorter time than a recording-layer crystallizing time to quenchthe random state to room temperature to form an amorphous recordingmark. On the other hand, the recording mark is erased by irradiating therecording layer with a light beam of an erase level to raise thetemperature of the recording layer to a temperature of less than itsmelting point and not less than its crystallizing temperature in alonger temperature raising time than the recording-layer crystallizingtime to crystallize the recording layer. In addition, the reproductionof information is carried out by utilizing the difference in reflectancebetween a crystal and an amorphous material.

[0004] Since the phase change recording medium can record whether itsstate is amorphous or crystalline before a recording operation, there isan advantage in that an overwrite operation can be carried out by onebeam.

[0005] As an example of the phase change recording medium, there is anoptical disc. A typical optical disc has a structure (a four-layerstructure) wherein a lower dielectric layer, a recording layer, an upperdielectric layer and a reflective layer are sequentially laminated on apolycarbonate substrate, the header part of which is pre-formatted andthe data part of which is pre-grooved. Moreover, a counter substrate isapplied to the reflective layer via an adhesive layer, or a label isapplied to the reflective layer.

[0006] As the recording layer, there is used a thin film of a chalcogenmetal compound, e.g., GeSbTe, AgInSbTe or InSbTe, which suitablyincludes a very small amount of Cr, V, N or the like.

[0007] The dielectric layer and the reflective layer serve to preventthe oxidation of the recording layer, to prevent the deterioration ofthe recording layer due to accumulated overwrite, to adjust the thermalresponse of the recording layer during a recording operation, and tooptically enhance the recording layer during a reproducing operation. Inparticular, with respect to the optical enhancement effect, the lowerdielectric layer can increase the variation in reflectance by themultiple interference effect between the substrate and the recordinglayer, and the upper dielectric layer can increase the variation inreflectance by the multiple interference effect between the recordinglayer and the reflective layer, so that the optical enhancement effectcan improve signal quality.

[0008] The phase change recording medium described above is applied tovarious information storage system, such as CD-RW (compactdisc-rewritable) and DVD-RAM (digital versatile disc-random accessmemory). In future, it is expected to increase the storage capacity ofthe phase change recording medium, to accelerate the transfer ratethereof, and to lower the price thereof.

[0009] However, the inventor has recognized that there are variousproblems to be solved in phase change recording media, after havingcontinued to make a unique study thereof. These problems will beenumerated below.

[0010] (Problem on Technique for Improving Transfer Rate)

[0011] First, the problem of transfer rates in the prior art will bedescribed. The request for the acceleration of data transfer rates ishigh similar to other recording media. However, in the phase changerecording media, it is required to shorten the time to crystallize arecording layer in order to improve the data transfer rate during arecording operation. Because the acceleration of the data transfer ratemeans the shortening of the time for an optical spot to pass. In orderto shorten the crystallizing time, it has been proposed to add a verysmall amount of an element other than principal elements constitutingthe recording layer to the recording layer, and/or to provide acrystallization controlling seed layer underlying the recording layer.However, this is not sufficient for the shortening of the crystallizingtime, so that the data transfer rate of the phase change recordingmedium is limited to tens Mbp (mega bit per second) or less.

[0012] (Problem on Reduction of Producing Costs)

[0013] Typically, a conventional method for producing a phase changerecording medium comprises:

[0014] (1) Master Disc Mastering Process;

[0015] (2) Stamper Producing Process;

[0016] (3) Substrate Forming Process by Injection;

[0017] (4) Film Attaching Process by Sputtering;

[0018] (5) (Bonding process of a counter substrate if necessary);

[0019] (6) Initial Crystallizing Process; and

[0020] (7) Verifying Process.

[0021] Among a series of these processes, “(6) Initial CrystallizingProcess” is a process for crystallizing an as-deposited phase changerecording layer (in a state as deposited) on the whole surface of adisc. The reason why this process is provided is that the as-depositedamorphous recording layer takes a very long time required to recordingunlike an amorphous mark formed by an optical recording operation.Therefore, the conventional phase change recording medium is not usedas-deposited, so that it is required to crystallize the recording layerat the initial crystallizing step.

[0022] For example, in the case of an optical disc, at the “initialcrystallizing step”, there is adopted a system for rotating a disc at arelatively low speed while irradiating the disc with elliptical laserbeams extending in radial directions of the disc at a high power to feedthe beams in radial directions at a shorter pitch than the major axes ofthe elliptical beams to gradually anneal the recording layer tocrystallize the recording layer. Although the time required for thecrystallization depends on the diameter of the disc, the linear velocityduring initialization, and the feed pitch, it takes at least severalminutes including the focusing time, so that the productivity is verybad. Since an actual producing line is designed so that a tact per discis several seconds, tens initializing systems must be arranged.Therefore, there are problems in that the costs for the systems arehigh, that it is required to ensure the area for installing the systems,that it is required to carry out the maintenance of the systems, thatthe productivity of the recording medium is low, and that the producingcosts increase.

[0023] (Problem on Degree of Freedom for Selection of Structure ofRecording Medium)

[0024] Another problem of conventional phase change recording media isthat the degree of freedom for the selection of the structure of themedium is limited. That is, although most of conventional phase changerecording media are set so that Rc (the light reflectance of a crystalpart) is higher than Ra (the light reflectance of an amorphous recordingmark), this results from the fact that it is required to carry out theinitial crystallizing step as described above.

[0025] That is, when the initial state of the medium is crystalline, theRc is set to be higher than the Ra, so that the reflectance beforerecording is high, the reflectance of address parts and data parts inthe initial state is high, the qualities of header signals and servosignals are improved, and the stability of servo is good.

[0026] However, if the limitation that the initial state of the mediumis crystalline is removed, the reflectance of the amorphous mark (Ra)can be freely designed so as to be higher or lower than the reflectanceof the crystal part by selecting the thickness and material of eachlayer.

[0027] However, since the conventional phase change recording medium hasa high Rc, there are disadvantages in that the absorptivity (Ac) of thecrystalline state can not be so high, so that the recording sensitivityis bad, that it is difficult to adjust the absorptivity required torecord the mark length, and that it is required to carry out theinitializing process, so that the costs for the producing process arehigh. The “adjustment of absorptivity” means that the absorptivity Ac ofthe crystalline state is set to be higher than the absorptivity (Aa) ofthe amorphous state in order to allow the film temperature of thecrystal part to be equal to that of the amorphous part during fusion inview of the latent heat of fusion, i.e., in order to reduce theoverwrite jitter.

[0028] In a medium having a so-called high to low structure (which willbe hereinafter briefly referred to as a “HtoL structure”) wherein Rc>Ra,there is no optical layer other than at least the recording layer, andAc<Aa is automatically established in the total reflection type filmstructure, so that it is not possible to adjust the absorptivity. Asmethods for adjusting the absorptivity in the HtoL structure whereinRc>Ra, there are methods for causing a reflective film to besemitransparent (thin), for providing a light absorbing layer betweenthe recording layer and the reflective layer, and so forth. However,there is a problem in that the ration Ac/Aa of the absorptivity is about1.2 at the most even by these methods, so that these methods are notsuitable for high linear velocity operations wherein it is required toadjust the absorptivity.

[0029] On the other hand, the LtoH (Low to High) medium, wherein the Rcis adjusted to be lower than the Ra, has the merits of having a highrecording sensitivity and being easy to adjust the absorptivity, so thatthe LtoH medium is expected to be the main current of optical discs inthe next generation. In particular, a medium having a five-layer filmstructure, wherein a thin semitransparent film of a metal is arrangedbetween the above described substrate of the four-layer film structureand a lower dielectric layer, can be designed so as to have an Ac/Aa of1.5 or more by suitably selecting the thickness of upper and lowerinterference films, and the crystal part thereof has a high recordingsensitivity, so that such a medium is suitable for high linear velocityoperations.

[0030] However, even in such an LtoH medium, the Rc decreases as theAc/Aa is set to be higher and as the reproducing CNR is set to behigher, so that there are problems in that it is difficult to readaddress parts if the initial crystallizing step is carried out similarto the medium having Rc>Ra, and that it is difficult to read the servosignals of data parts in a state before recording.

[0031] (Problem on Increase of Storage Capacity)

[0032] As techniques for improving the recording density of a phasechange medium, there are techniques for decreasing the wavelength of alight source, for increasing the NA of an objective lens, for applying asuper resolution thin-film and so forth. On the other hand, as means forimproving the storage capacity of the medium without the need of theimprovement of the recording density of the medium, a single-sideddouble-layer disc is provided. The single-sided double-layer disc isdesigned to record and/or reproduce data by only adjusting the focalposition of light beams on a double-layer recording layer apart from theplane of incidence for the same light beam by about tens μm. Since it isnot required to turn a disc over, it is considered by the user that thesingle-sided double-layer disc has substantially the same performance asthat of a single-sided single-layer disc having a recording densitysubstantially twice as large as that of the single-sided double-layerdisc. As a reproduction only DVD, there is known a single-sideddouble-layer disc which is known as a common name DVD-9. However, it hasbeen considered that since the transmittance of a rewritable DVD isinsufficient by one recording layer, light beams do not sufficientlyreach a recording layer arranged at the bottom with respect to theincident side of light beams, so that it is difficult to record and/orreproduce data.

[0033] However, in ISOM (International Symposium on Optical Memory) '98,Technical Digest, pp. 144-145 (Th-N-05), it has been suggested that itis possible to form a single-sided double-layer even in the case of arewritable phase change medium. The points of this technique are thatthe transmittance of a first recording layer part is increased to about50% so that light beams sufficiently reach a second recording layer partarranged at the bottom when the first recording layer part and thesecond recording layer part are arranged in that order from the incidentside of light beams, that the reflectance of the second recording layerpart is set to be higher, i.e., the transmittance thereof is lower, inorder to maintain the balance of servo signals and regenerative signalsfrom the first and second recording layer parts, and that theabsorptivity Ac of the crystal part is set to be higher than theabsorptivity Aa of the amorphous part in both the first and secondrecording layer parts in order to reduce overwrite jitters.

[0034] In order to satisfy the above described setting, the firstrecording layer part has a three-layer construction which has aso-called High to Low structure (which will be hereinafter brieflyreferred to as a “HtoL structure”), wherein the reflectance Rc of thecrystal part is higher than the reflectance Ra of the amorphous part,and which has no reflective film, and the second recording layer parthas a five-layer construction which has the LtoH structure, wherein thereflectance Rc of the crystal part is lower than the reflectance Ra ofthe amorphous part, a thin Au semitransparent film underlying the LtoHstructure, and a thin Al—Cr reflective film on the top of the LtoHstructure.

[0035] In this construction, with respect to the reflectance of eachrecording layer part viewed from the incident side of light beams, thereflectance of the first recording layer part is 9% of that of thecrystal part and 2% of that of the amorphous part, and the reflectanceof the second recording layer part is about 3% of that of the crystalpart and about 9% of that of the amorphous part. Therefore, if thesingle-sided double-layer phase change medium is initial-crystallized inaccordance with the conventional producing process, the initialreflectance of the address part and data part is about 9% in the firstrecording layer and about 3% in the second recording layer. This initialreflectance is far lower than, e.g., 15% to 25% of the single-sidedsingle-layer DVD-RAM standard. At the initial reflectance of the firstrecording layer, it is possible to reproduce address signals and servosignals of the data part somehow if the reproducing power is increased.However, the reflectance of the second recording part is too low, sothat it is difficult to reproduce both of address signals and servosignals.

[0036] In addition, the common problem of single-sided double-layermedia, which are not limited to the above described rewritable media, isthat the initial crystallizing step is complicated. That is, if each ofthe first and second recording layer parts is initial-crystallized, itis required to carry out double steps to deteriorate the productivityand producing costs.

SUMMARY OF THE INVENTION

[0037] The present invention has been made on the basis of therecognition of the aforementioned problems. That is, it is a principalobject of the present invention to improve the recording transfer rateof a phase change recording medium, to reduce the producing costs of themedium, to greatly increase the degree of freedom for selecting thestructure of the medium to particularly provide a structure of Rc<Ra,and to increase the storage capacity of the medium.

[0038] More specifically, it is an object of the present invention toshorten the time required to crystallize a recording layer to improve adata transfer rate, to omit an initial crystallizing step to reduce theproducing costs by applying a rapid crystallizing performance to anas-deposited amorphous material, to allow a medium of Rc<Ra to be usedin an as-deposited amorphous state to extend the range of selecting thestructure of the medium, and to improve the reflectance of asingle-sided double-layer medium to increase the storage capacitythereof.

[0039] That is, according to the present invention, a phase changerecording medium has a first recording layer wherein a phase changebetween an amorphous state and a crystalline state occurs reversibly bylight irradiation to change the optical characteristic of the firstrecording layer, the state of the recording layer being the amorphousstate before a recording operation is carried out, and the recordinglayer containing fine nuclei having a grain size of from 0.5 nm to 4 nm.

[0040] The first recording layer may be irradiated with erasing lightbeams to produce a crystal part, and the distribution of the number ofcrystal grains constituting the crystal part with respect to the grainsizes of the crystal grains may have maximum values with respect to atleast two different grain sizes.

[0041] In the phase change recording medium a grain size at a firstmaximum value of the at least two different maximum values may begreater than 4 nm and 20 nm or less, a grain size at a second maximumvalue of the at least two different maximum values may be greater than20 nm and 100 nm or less, and the percentage of the sum of grain sizesbelonging to a distribution, the center of which is the first maximumvalue, and grain sizes belonging to a distribution, the center of whichis the second maximum value, in all of the crystal grains of the crystalpart may be 75% or higher.

[0042] The phase change recording medium may further comprise: a secondrecording layer wherein a phase change between an amorphous state and acrystalline state occurs reversibly by light irradiation to change anoptical characteristic; and a separation layer provided between thefirst recording layer and the second recording layer.

[0043] According to the present invention, a phase change recordingmedium has a first recording layer wherein a phase change between anamorphous state and a crystalline state occurs reversibly by lightirradiation to change the optical characteristic of the first recordinglayer, the distribution of the number of crystal grains constituting therecording layer with respect to the grain sizes of the crystal grainshaving maximum values with respect to at least two different grain sizesin the amorphous state of the recording layer.

[0044] In the phase change recording medium, a grain size at a firstmaximum value of the at least two different maximum values may begreater than 4 nm and 20 nm or less, a grain size at a second maximumvalue of the at least two different maximum values may be greater than20 nm and 100 nm or less, and the percentage of the sum of grain sizesbelonging to a distribution, the center of which is the first maximumvalue, and grain sizes belonging to a distribution, the center of whichis the second maximum value, in all of the crystal grains of the crystalpart may be 75% or higher.

[0045] The first recording layer may be irradiated with recording lightbeams to produce an amorphous part containing fine nuclei having a grainsize of from 0.5 nm to 4 nm.

[0046] The phase change recording medium may further comprise: a secondrecording layer wherein a phase change between an amorphous state and acrystalline state occurs reversibly by light irradiation to change anoptical characteristic; and a separation layer provided between thefirst recording layer and the second recording layer.

[0047] According to the present invention, a phase change recordingmedium has a recording layer wherein a phase change between an amorphousstate and a crystalline state occurs reversibly by light irradiation tochange the optical characteristic of the recording layer, the state ofthe recording layer being the amorphous state before a recordingoperation is carried out, and the recording layer having a thermalconductivity of from 0.8 W/mK to 6 W/mK.

[0048] The recording layer may contain fine nuclei having a grain sizeof from 0.5 nm to 4 nm.

[0049] According to the present invention, a phase change recordingmedium has a recording layer wherein a phase change between an amorphousstate and a crystalline state occurs reversibly by light irradiation tochange the optical characteristic of the recording layer, the recordinglayer containing at least one of Kr and Xe in the range of from 0.2 at %to 10 at %.

[0050] According to the present invention, a phase change recordingmedium has a recording layer wherein a phase change between an amorphousstate and a crystalline state occurs reversibly by light irradiation tochange the optical characteristic of the recording layer, the recordinglayer having an amorphous state band part between adjacent tracks afterthe recording layer is irradiated with a recording light beam which hasa spot size of a e⁻² diameter greater than a track pitch.

[0051] The state of the phase change recording medium may be anamorphous state wherein the address part of the recording layer hassubstantially the same randomness as that of the amorphous recordingmark of the data part.

[0052] More specifically, the state of the address part may be anamorphous state, and the recording layer may contain fine nuclei havinga grain size of from 0.5 nm to 4 nm.

[0053] In addition, the state of the address part of the recording layermay be amorphous state, and the recording layer may have a thermalconductivity of from 0.8 W/mK to 6 W/mK.

[0054] According to the present invention, there is provided a methodfor producing a phase change recording medium having a substrate and arecording layer deposited on the substrate, wherein the relationshipbetween a dc voltage Vdc applied to a target and a sputter thresholdvoltage Vth of a target constituting element is set to be Vth<Vdc≦10 Vthwhen a recording layer of a phase change recording medium is depositedby sputtering.

[0055] In this process, an ion density Ni in a negative glow plasmaproduced in the vicinity of the target during the sputtering may be inthe range of Ni>10¹¹ (cm⁻³).

[0056] According to the present invention, there is provided a methodfor producing a phase change recording medium having a substrate and arecording layer deposited on the substrate, wherein while or after therecording film is deposited on the substrate, nuclei are produced in therecording film by raising the temperature of the recording film to ahigher temperature than room temperature while the temperature of thesubstrate is maintained to be less than the thermal deformationtemperature thereof.

[0057] In this method, the temperature rise may be carried out by aninfrared ray lamp.

[0058] In this method, the temperature rise may be carried out while thesubstrate is supported on a material which does not substantially absorblight beam emitted from the infrared ray lamp.

[0059] According to the present invention, there is provided a methodfor producing a phase change recording medium, which has a firstrecording layer part wherein a phase change between an amorphous stateand a crystalline state occurs by light irradiation, a separation layerformed on the first recording layer part, and a second recording layerpart which is formed on the separation layer and wherein a phase changebetween an amorphous state and a crystalline state occurs, wherein theinitial crystallization of the first recording layer part and theinitial crystallization of the second recording layer part aresubstantially simultaneously carried out.

[0060] In this method, the initial crystallization may be carried out byirradiating with initial crystallizing light beams, and part of theinitial crystallizing light beams for irradiating the first recordinglayer part may be used for the initial crystallization of the recordinglayer part.

[0061] The method for producing a phase change recording medium mayfurther comprise the steps of: depositing the first recording layer parton the first substrate; depositing the second recording layer part onthe second substrate; and sticking the first and second substratestogether via a separation layer after the initial crystallizing step sothat the first and second recording layer parts deposited sides faceeach other.

[0062] According to the present invention, a system for producing aphase change recording medium having a substrate and a recording filmdeposited on the substrate, which has heating means for raising thetemperature of the recording film to a higher temperature than roomtemperature while maintaining the temperature of the substrate to beless than the thermal deformation temperature thereof, to produce nucleiin the recording film, while or after the recording film is deposited onthe substrate, nuclei are produced in the recording film.

[0063] The heating means may be an infrared ray lamp.

[0064] The system may further comprise a substrate holder for supportingthe substrate, the contact portion of the substrate holder with thesubstrate being made of a material which does not substantially absorblight beams emitted from the infrared ray lamp.

[0065] According to the present invention, a system for producing aphase change recording medium comprises: a first holding part forholding a first substrate, on which a first recording layer part whereina phase change between a crystalline state and an amorphous state occursby light irradiation is deposited; a second holding part for holding asecond substrate, on which a second recording layer part wherein a phasechange between a crystalline state and an amorphous state occurs bylight irradiation is deposited; a light irradiation part for irradiatingwith an initial crystallizing light beam for initial-crystallizing thefirst and second recording layer parts; and an optical system forcondensing the initial crystallizing light beam passing through thefirst recording layer part on the second recording layer part toirradiate the second recording layer part with the initial crystallizinglight beam.

[0066] According to the present invention, with the above describedconstructions, it is possible to improve the recording transfer rate ofthe phase change recording medium, and it is possible to reduce theproducing costs of the medium. In addition, it is possible to greatlyincreasing the degree of freedom for the selection of the structure ofthe medium, particularly it is possible to provide a medium having astructure of Rc<Ra, so that it is possible to increase the storagecapacity of the medium.

[0067] More specifically, it is possible to reduce the time required tocrystallize the recording layer to improve the data transfer rate, andit is possible to omit the initial crystallizing step to reduce theproducing costs by applying a rapid crystallization performance to theas-deposited amorphous material. Moreover, by allowing the medium ofRc<Ra to be used from the as-deposited amorphous state, it is possibleto increase the range for selecting the structure of the medium, and itis possible to improve the reflectance of a single-sided double-layermedium, so that it is possible to increase the storage capacity of themedium.

BRIEF DESCRIPTION OF THE DRAWINGS

[0068] The present invention will be understood more fully from thedetailed description given herebelow and from the accompanying drawingsof the preferred embodiments of the invention. However, the drawings arenot intended to imply limitation of the invention to a specificembodiment, but are for explanation and understanding only.

[0069] In the Drawings:

[0070]FIGS. 1A through 1C are schematic diagrams of the fine structureof a recording layer of a preferred embodiment of a phase changerecording medium according to the present invention, in comparison witha conventional phase change recording medium, wherein FIG. 1A shows thefine structure of a recording layer in this preferred embodiment, FIG.1B showing the fine structure of a recording layer in an as-depositedstate produced by the prior art, and FIG. 1C showing the fine structureof a crystal part formed by an optical recording operation in arecording layer produced by this preferred embodiment and the prior art;

[0071]FIG. 2 is a sectional view of a phase change recording mediumaccording to the present invention;

[0072]FIG. 3 is a graph showing the relationships between the number ofOW repetitions and CNR (carrier to noise ratio), which were measuredwith respect to an as-deposited phase change medium formed in accordancewith the present invention and an as-deposited (a part which is notinitial-crystallized) phase change medium formed in accordance with theprior art;

[0073]FIG. 4 is a schematic diagram showing an example of a TEM image ofa recording layer of a part between marks of an optical disc accordingto the present invention;

[0074]FIG. 5 is a graph showing the distribution in crystal grain sizeof crystal grains having a greater size than 20 nm in a part betweenmarks of an optical disc according to the present invention;

[0075]FIG. 6 is a graph showing the distribution in crystal grain sizeof fine crystal grains of an optical disc according to the presentinvention;

[0076]FIG. 7 is a conceptual sectional view of the first preferredembodiment of a phase change optical recording medium according to thepresent invention;

[0077]FIG. 8 is a graph wherein appearance frequencies of measuredrespective grain sizes are plotted in a comparative example;

[0078]FIG. 9 is a schematic sectional diagram showing an example of thethird preferred embodiment of a phase change optical disc according tothe present invention;

[0079]FIG. 10 is a schematic diagram of a principal part of a sputteringsystem for use in the third preferred embodiment;

[0080]FIG. 11 is a graph showing the relationship between the amounts ofKr, which are contained in a recording layer, and 3T jittercharacteristics;

[0081]FIG. 12 a schematic sectional view of a second example of thethird preferred embodiment of an optical disc according to the presentinvention;

[0082]FIG. 13 is a schematic diagram showing a pattern after the firstrecording operation in the optical disc in the second example;

[0083]FIG. 14 is a schematic diagram showing a pattern during thehundredth overwrite (OW100) of the optical disc in the second example;

[0084]FIG. 15 a conceptual sectional view showing an example of thefourth preferred embodiment of a phase change recording medium accordingto the present invention;

[0085]FIG. 16 is a conceptual sectional view showing an example of thefourth preferred embodiment of a phase change recording medium accordingto the present invention;

[0086]FIG. 17 is a conceptual plan view showing an example of arecording medium;

[0087]FIG. 18 is a graph showing the relationship between the measuredvalues of thermal conductivity (κ) and DC erasing rates measured using adisc sample having the structure of FIG. 15;

[0088]FIG. 19 is a graph showing the relationship between the values of3T-CNR and the values of thermal conductivity in the first recordingoperation without initialization;

[0089]FIG. 20 is a graph showing an example of the distribution incrystallizing time in a GeSbTe ternary alloy system;

[0090]FIG. 21 is a graph showing an example of the distribution incrystallizing temperature in a GeSbTe ternary alloy system;

[0091]FIG. 22 is a graph showing a desired composition range in anAg—In—Sb—Te four-element alloy;

[0092]FIG. 23 is a conceptual diagram of a magnetron sputtering systemfor use in the fifth preferred embodiment of the present invention;

[0093]FIG. 24 is a conceptual sectional view of a recording mediumexperimentally manufactured in an example of the fifth preferredembodiment;

[0094]FIG. 25 is a graph showing the relationship between the values ofVdc/Vth, non-initialized first-recording characteristics, and thin-filmdeposition rates;

[0095]FIG. 26 is a conceptual sectional view of a phase change recordingmedium produced in an example of the fifth preferred embodiment;

[0096]FIG. 27 is a conceptual diagram showing an example of a phasechange recording film forming system for use in the sixth preferredembodiment of the present invention;

[0097]FIG. 28 is a graph showing the evaluated results of discs;

[0098]FIG. 29 is a conceptual diagram showing an example of a substrateholder which does not absorb lamp rays;

[0099]FIG. 30 is a graph showing the evaluated results of the values ofCNR and noise levels;

[0100]FIG. 31 is a schematic sectional view of a first example of theseventh preferred embodiment of a phase change optical disc according tothe present invention;

[0101]FIG. 32 is a schematic sectional view of a second example of theseventh preferred embodiment of a phase change optical disc according tothe present invention; and

[0102]FIG. 33 is a schematic sectional view of a producing system forinitial-crystallizing a third example of the seventh preferredembodiment of a phase change optical disc according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0103] Referring now to the accompanying drawings, the preferredembodiments of the present invention will be described below.

[0104] (First Preferred Embodiment)

[0105] First, the first preferred embodiment of the present inventionwill be described. This preferred embodiment is characterized that theinitial state, i.e., the as-deposited state, of a phase change recordingmedium, is an amorphous state which has a unique short-range orderstructure.

[0106]FIGS. 1A through 1C are schematic diagrams showing the finestructure of a recording layer of a phase change recording medium inthis preferred embodiment, in comparison with a conventional phasechange recording medium. That is, FIG. 1A is a schematic diagram showingthe fine structure of a recording layer in this preferred embodiment,and FIG. 1B is a schematic diagram showing the fine structure of arecording layer in the as-deposited state, which is produced by theprior art. In addition, FIG. 1C is a schematic diagram showing the finestructure of a crystal part formed by an optical recording operation ina recording layer produced by this preferred embodiment and prior art.

[0107] The as-deposited state of the recording layer produced by theprior art (FIG. 1B) shows a random arrangement which has neither along-range order nor a short-range order. On the other hand, althoughthe recording layer in the as-deposited state produced by this preferredembodiment has no long-range order, it has a short-range order having asize of 0.5 nm (a hyperfine nucleus of about 8 atoms) to 4 nm (a finenucleus of about 4000 atoms) (this is shown by A in FIG. 1A). Inaddition, the inventor observed that the same short-range order A existsin an amorphous part formed by an optical recording operation in arecording layer produced by the present invention and the prior art.

[0108] On the other hand, it was observed that a polycrystalline statehaving a long-range order of about 20 nm or more exists in the crystalpart formed by the optical recording operation in a recording layerproduced by this preferred embodiment and the prior art.

[0109] The recording layer shown in FIG. 1A is able to be produced byvarious means which will be described below. Before describing examplesof this preferred embodiment, a basic concept for producing a recordinglayer of a phase change recording medium in this preferred embodimentwill be described below.

[0110] A recording film for use in a phase change recording is usuallydeposited by a sputtering method, and the state thereof is an amorphousstate immediately after the thin-film deposition. The sputtering methodis a technique for producing a predetermined film by allowing thetransition of the surface of a substrate into a solid state serving as afilm after sputter particles (gaseous phase), which are sputter-emittedfrom a target surface by a high-energy argon (Ar) ion bombardment,arrive at the surface of the substrate at random to migrate the surfacein a random state.

[0111] It is generally said that a sputter particle has an energy ofseveral eV. The value of 1 eV corresponds to 10⁴ K, which is far higherthan thermal energy at room temperature. In addition, the transitionrate of a sputter particle from a gaseous phase into a solid phase on asubstrate is usually about 10¹² k/sec. That is, it is guessed that thetime required for a sputter particle to change from a random state ofseveral eV (tens of thousands K) into a solid phase at room temperatureis about 10 nanoseconds, and the time required for the sputter particleto pass through a temperature zone between the melting point and thecrystallized temperature is about 1 nanosecond at the most. On the otherhand, the crystallization holding time required to crystallize a GeSbTeor InSbTe recording film is tens nanoseconds. That is, when a film isdeposited by a conventional sputtering method, the sputter particle iscooled in a far shorter time than the crystallization holding time, sothat the state of the recording layer is an amorphous state having noshort-range order immediately after the sputter deposition as shown inFIG. 1B.

[0112] This amorphous state immediately after the thin-film depositionis different from an amorphous state formed by an optical recordingoperation. Because the cooling rate during the optical recordingoperation is typically about 10¹⁰ K/sec, which is lower than that duringa sputter deposition by about two figures, although it depends on thelinear velocity and the layer structure of a medium. That is, since thecooling rate in a film deposition process during the sputter depositionis very high which is higher than the cooling rate during the formationof an amorphous part by about two figures, it is considered that anamorphous material having a higher randomness than that of the amorphouspart after an optical recording operation, i.e., an amorphous materialhaving a smaller short-range order, is formed in an as-deposited film.

[0113] If the quality of the amorphous state immediately after thesputter deposition is the same as that of the amorphous state formed bythe optical recording operation, it is possible to carry out therecording and reproducing operations without carrying out the initialcrystallizing step. However, since the amorphous state immediately afterthe sputter deposition is different from the amorphous state formed bythe optical recording operation, it is difficult to carry out theoptical recording in either a medium of Rc>Ra or a medium of Rc<Rawithout carrying out the initial crystallizing step. The inventor foundthat the amorphous state formed by the optical recording operation has ashort-range order although it is an amorphous state at a long distancesince the crystallization holding time in a cooling process approximatesto the time to crystallize the recording layer. Moreover, the sputteringprocess for a recording layer and interference layers arranged above andbelow the recording layer is improved on the basis of the abovedescribed finding, so that the inventor succeeded in forming ashort-range order, which is the same as that in the amorphous stateformed by the optical recording operation, in an as-deposited amorphousstate to make the present invention.

[0114] Specifically, according to the present invention, one process isto decrease the cooling rate for sputter particles in a sputteringprocess to cause an amorphous state immediately after a sputterdeposition to approximate to an amorphous state formed by an opticalrecording operation, and another process is to apply a compressiblestress to a recording layer immediately after a sputter deposition toprovide a state that the recording layer is easily crystallized. Theseprocesses may be combined.

[0115] In order to cause the amorphous state immediately aftersputtering to approximate to the amorphous state formed by the opticalrecording operation, the energy of sputter particles being incident on asubstrate is decreased, and/or the surface migration time is controlledso as to increase. Specifically, there are effective methods for usingKr (krypton) or Xe (xenon) gas, which are capable of conspicuouslycooling GeSbTe sputter particles, or the mixed gas thereof as a sputtergas in place of Ar gas typically used, and/or for applying a bias to asubstrate to promote the surface migration. The state of the thus formeddisc immediately after the sputter deposition approximates to theamorphous state formed by the optical recording operation, and is thesame as the amorphous state at a long distance, but it has a finestructure having a short-range order.

[0116] On the other hand, in order to apply a compressive stress to arecording layer immediately after the sputter deposition to promote theformation of the short-range order, it is effective to enhance thecompressive stress of the recording layer itself or to apply acompressive stress to the interference layers arranged above and belowthe recording layer to allow the recording layer to easily contract. Thevolume of the recording layer in the amorphous state having theshort-range order is slightly smaller than that in the amorphous statehaving no short-range order. Therefore, if the compressive stress isapplied, it is easy to cause the volume of the recording layer tocontract, i.e., to form the short-range order. In the case of a sputterfilm, it is possible to apply the compressive stress by causing highenergy particles being incident on the substrate during the thin-filmdeposition. Specifically, it is effective to apply a bias to thesubstrate to increase the energy of sputter particles being incident onthe substrate, or to apply a bias to the substrate to accelerate gasions to positively cause the gas ions to be incident on the substrate.

[0117] Among these methods, the method for decreasing the gas pressureduring the formation of the recording layer collides with the firstmethod for decreasing the energy of sputter particles. However, theconditions for reasonably decreasing the cooling rate for sputterparticles on the substrate to apply a moderate compressive stress to therecording layer are suitable for the present invention, and the methodfor decreasing the gas pressure during the formation of the recordinglayer is found by the relationship with other methods. However, therecording film is most preferably formed by carrying out a sputterdeposition on a high pressure condition using a heavy rare gas (thissputter deposition is suitable for the application of a compressivestress at the same time that a bias is applied to a substrate), tosputter a low pressure gas to apply a great compressive stress when theupper and lower interference films are formed. The foregoing is a basicconcept of a method for producing a phase change recording mediumaccording to the present invention.

[0118] The present invention will be described in detail below.

[0119]FIG. 2 is a sectional view of a phase change recording mediumaccording to the present invention. In FIG. 2, reference number 101denotes a disc substrate, 102 denoting a semitransparent layer, 103denoting a lower interference layer, 104 denoting a recording layer, 105denoting an upper interference layer, and 106 denoting a reflectivelayer. The substrate 101 is a polycarbonate substrate having a diameterof 120 mm and a thickness of 0.6 mm. The substrate 101 comprises anaddress part (not shown) comprising pre-pit strings, and a data part(not shown) wherein-pre-grooves are formed. A method for producing asubstrate comprises the typical steps of: mastering a master disc,preparing a stamper by plating, and injection-forming a polycarbonateresin on the stamper. In this example, the track width of each of bothof grooves and lands was set to be 0.74 μm, which was the same as thatof the first generation DVD-RAM standard, and the depth of each of thegroove was set to be 70 nm, which was the same as that of the crosstalkspecification.

[0120] The phase change medium formed on the substrate comprises an Ausemitransparent film 102 having a thickness of 10 nm, a bottom ZnS—SiO₂interference film 103 having a thickness of 85 nm, a GeSbTe recordinglayer 104 having a thickness of 10 nm, a top ZnS—SiO₂ interference film105 having a thickness of 30 nm, and an AlMo reflective film 106 havinga thickness of 100 nm, which are arranged in that order from thesubstrate. The composition of each of the upper and lower ZnS—SiO₂interference films was a standard composition containing 20 at. % SiO₂,and the composition of the GeSbTe recording layer was a standardcomposition wherein Ge:Sb:Te=2:2:5.

[0121] The optical design values of a disc having the above describedfilm structure include an Rc of 5%, an Ra of 20%, and an Ac/Aa of 1.3,which are the same as those of a typical absorptivity adjusting LtoHstructure. In addition, in the above describe film structure, the phasedifference of reflected light was adjusted to be zero. All of layersother than the recording layer were formed by typical sputtering methodand conditions for use in typical experiments. That is, the sputteringmethod was the magnetron sputtering method, and the sputter gas was pureAr gas. In addition, the gas pressure was 0.67 Pa, and the powerinputted to the target was tens to hundreds W. Moreover, the substratewas a non-bias substrate. In addition, the technique and conditions fordepositing the GeSbTe film was special technique and conditions forobtaining a disc according to the present invention. An example of amethod for forming a recording layer will be described below.

[0122] First, a substrate 101, on which a semitransparent layer 102 anda lower interference layer 103 have been formed, is introduced into asputter chamber for forming a recording layer 104, and secured to asubstrate holder facing a target. Then, the sputter chamber isevacuated, and a Kr—Xe mixed gas containing 20% Kr is introduced from agas supply system at a flow rate of 200 sccm. Then, after theconductance of an exhaust system 7 is adjusted so that the gas pressurein the sputter chamber is adjusted to be 6.7 Pa, an RF power of 50 W isinputted to a sputter source by a power source 5, and simultaneously, asurface migration control system is operated to input a weak RF power tothe substrate to bias-magnetron-sputter a GeSbTe target for five minutesto form a GeSbTe recording layer 104 having a thickness of 10 nm on thelower interference layer.

[0123] The different points from a typical sputtering were as follows:the Kr—Xe mixed gas was used as a sputter gas to easily lose GeSbTesputter particles in a gaseous phase; the gas pressure was set to behigh to promote the cooling of the GeSbTe sputter particles in thegaseous phase, and the energy of the gas ions being incident on thetarget was decreased to decrease the energy when the sputter particleswere emitted from the target; the power inputted to the sputter sourcewas set to be relatively low to more decrease the energy when thesputter particles were emitted; and the weak bias was applied to thesubstrate to cause the gas ions to be incident on the substrate duringthe thin-film deposition so as not to thermally damage the polycarbonatesubstrate to increase the time to surface migrate the sputter particleson the substrate.

[0124] The above described points are effective in the decrease of theenergy of the sputter particles being incident on the substrate and inthe increase of the transition time from a random liquid phase state toa solid phase state on the substrate, i.e., in the decrease of thecooling rate for the sputter particles. By such a method, the coolingrate for the GeSbTe sputter particles in the sputter deposition processcan be decreased from 10¹² K/sec in the conventional method to an orderof 10¹⁰ K/sec during an optical recording operation, so that ashort-range order having a size of 0.5 nm to 10 nm can be formed in anamorphous state immediately after the thin-film deposition similar tothe amorphous state during the optical recording operation. In order tocause the amorphous state immediately after the thin-film deposition toapproximate to the amorphous state during the optical recordingoperation, it is not always required to carry out all of the selectionof the sputter gas suitable for the above described material of therecording layer, the increase of the gas pressure, the decrease of thesputter power, and the application of the substrate bias, and these maybe suitably combined. In addition to the above described methods, thereare a method for promoting the surface migration of the sputterparticles on the substrate by a method for heating the substrate byabout tens ° C. during the thin-film deposition or by a method forproviding an auxiliary ion source to irradiate the surface of thesubstrate with an ion shower; and a method for decreasing the energy ofthe sputter particles being incident on the substrate by a method forionizing the sputter particles emitted from the target surface to passthe sputter particles through a deceleration field, a method forincreasing the distance between the target and the substrate, or amethod for eccentrically arranging the substrate with respect to thetarget to deposit a film by only sputter particles emitted obliquelyfrom the target surface. By suitably combining these methods, theamorphous state immediately after the sputter deposition can approximateto the amorphous state during the optical recording operation.

[0125] While the recording layer 104 has been deposited by the mostpractical sputtering method in all of the above described examples, itis effective to apply the vacuum deposition method, the gas depositionmethod, the MBE (molecular beam expitaxy) method, the plasma CVD(chemical vapor deposition) method, the MOCVD (metal-organic chemicalvapor deposition) method, or the like to the deposition of the recordinglayer in order to set the energy of the recording film materialparticles being incident on the substrate to be low.

[0126] On the GeSbTe recording film 104 produced by the substrate biasmagnetron sputter method at a high pressure and low power in the abovedescribed Kr—Xe gas, an upper interference layer 105 and a reflectivelayer 106 are sequentially formed by a typical magnetron sputteringmethod to be ejected from the sputter chamber. The five-layer disc thusformed is applied on a polycarbonate substrate having a diameter of 120mm and a thickness of 0.6 mm, on which no film is provided, via aUV-curing adhesive layer, to be prepared as a sample for verifying theadvantages of the present invention.

[0127] As a comparative example, a disc having the same filmconstruction as that in the above described example was prepared by adifferent producing method. The comparative disc was formed byinitial-crystallizing a part of a recording layer with elliptical beamsin accordance with a conventional producing process after the recordinglayer was formed on typical sputtering conditions by a typicalsputtering method to form an applied structure. The conditions forsputtering the recording layer for use in the comparative example werethe same as the conditions for sputtering the films other than therecording layer in the above described example of the present invention.That is, the magnetron sputtering method was used, and the sputter gaswas pure Ar gas. In addition, the gas pressure was 0.67 Pa, and thepower inputted to the target was tens to hundreds W. Moreover, thesubstrate was a non-bias substrate.

[0128] The initial crystallizing parts of the disc of the presentinvention and the comparative disc were evacuated by the followingmethod. First, a disc evaluating system having a recording and/orreproducing optical system having a wavelength of 650 nm and an NA ofobjective lens of 0.6 was used to measure the reflectance of the mirrorsurface of the address part. Then, with respect to the data part, a 8/16modulating random pattern signal having a linear velocity of 6 m/sec anda shortest bit length of 0.41 μm/bit was recorded on adjacent land andgroove, each of which has 10 tracks, and a regenerative signal wasdetected using a time interval analyzer to measure a jitter ratio (%) toa window width. The measurement of the jitter was carried out withrespect to each of the land track and the groove track when each of thefirst recording operation, the tenth overwrite operation and thehundredth overwrite operation was carried out. The above describedevaluating Conditions are based on the first generation DVD-RAM standarddefining that the reflectance of the mirror part is 15% or more and theallowable jitter amount is 8.5% with respect to random data. Theevaluated data are shown in table 1. TABLE 1 Evaluated ResultsReflectance (%) Jitter (%) of Mirror Surface of Data Part Disc ofAddress Part 1st L/G OW10 L/G OW100 L/G Present 20 7.5/7.2 7.7/7.37.3/7.0 Invention Comparative 5 7.3/7.0 7.5/7.2 7.2/7.0 Example

[0129] As can be clearly seen from table 1, the disc of the presentinvention has a high reflectance of the mirror surface and an excellentaddress signal quality. In addition, the reflectance of the data part ishigh from the first time, and the jitter of the data part is low fromthe first recording operation, so that it is possible to carry out agood recording.

[0130] On the other hand, in the case of the comparative disc producedby the conventional method, the whole surface is initial-crystallized.Therefore, although the jitter characteristic of the data part has agood value from the first recording operation, the reflectance of themirror part is low, and the reflectance of the data part during thefirst recording operation is also low, so that the qualities of both ofaddress signals and tracking servo signals are lower than those of adisc according to the present invention. In addition, as another greatadvantage of the present invention, it is possible to omit the initialcrystallizing step from the producing process, so that it is possible toprovide inexpensive discs. This needs no explanation.

[0131] The fine structures of a disc of the present invention and acomparative disc after recording were observed by the transmissionelectron microscope (TEM). In order to carry out the TEM observation, anas-deposited part and a recording part (optically recorded amorphous andcrystal parts) were cut to prepare a small piece to peel away and removethe counter substance along with the UV adhesive layer. Thereafter, thepeeled small piece was embedded in a resin to be polished, so that thesectional part of the medium film was exposed. Then, the recording layerof the sectional part of the medium film was observed by a highresolution electron microscope.

[0132] The observed results are shown in FIG. 1 described above. Thatis, FIG. 1A shows the fine structure of the as-deposited state of arecording layer 104 of a phase change medium according to the presentinvention and an amorphous mark part formed by an optical recordingoperation. FIG. 1C shows the state of a regular atomic arrangement of acrystal space formed by an optical recording operation of the recordinglayer 104 according to the present invention. FIG. 1B shows the finestructure of the as-deposited state (a part which is notinitial-crystallized) of a phase change medium in a comparative exampleprepared by the prior art. The state of the phase change medium preparedby the prior art after the initial crystallization, and state of thecrystal part formed by the optical recording operation were the same asthose in FIG. 1C. In addition, the fine structure of the amorphous partformed by the optical recording operation on the initial-crystallizedpart of the phase change medium produced by the prior art was the sameas that in FIG. 1A.

[0133] Then, the relationship between the number of OW repetitions andCNR (carrier to noise ratio) was measured with respect to anas-deposited phase change medium, which was formed in accordance withthe present invention, and an as-deposited (a part which is notinitial-crystallized) phase change medium which was formed in accordancewith the prior art. FIG. 3 is a graph showing the measured result. Ascan be clearly seen from FIG. 3, in the phase change recording mediumformed in accordance with the present invention (which is expressed by“Present Invention” in FIG. 3), a high CNR exceeding 52 dB was obtainedover the OW repetitions from the first recording operation after theas-deposition, whereas in the medium formed in accordance with the priorart (which is expressed by “Comparative Example” in FIG. 3), the CNR islow, about 20 dB, at the first recording operation after theas-deposited state, and it is not possible to obtain substantially thesame CNR as that a non-initialized medium unless OW-is repeated about100 times. It is considered that the recording medium of the presentinvention is amorphous at a long distance, but it has a short-rangeorder, so that the short-distance serves as a nucleus to achieve a goodrecording after the as-deposited state.

[0134] Then, the relationship between the CNR in the first as-depositedstate and the size of the short-range order was examined. The size ofthe short-range order was controlled by adjusting the above-describedprocess for sputtering the recording layer and the upper and lowerinterference layers. For example, as the gas pressure during thethin-film deposition of the recording layer was increased, as thesputter input to the target was decreased, and as the bias voltageapplied to the substrate was increased, the size of the short-distancewas increased. As a result, it was found that a cluster having about 8atoms is required to exist in order to obtain a good CNR from the firstrecording operation in the as-deposited state. By the observation usingthe high resolution electron microscope, it was also found that the sizeof the cluster was 0.5 nm. When the short-range order (fine nucleus) was5 nm or more, i.e., when about 8000 atoms are regularly arranged, thedistance between clusters in the short-range order was decreased, andthe fine structure having the short-range orders scattered in theamorphous material was changed to a polycrystalline structure. Such apolycrystalline structure is similar to the structure after the processfor initial-crystallizing the phase change medium produced by the priorart, or the structure of the crystal part formed by the opticalrecording operation. That is, it was found that if the polycrystallinestructure was applied to an LtoH medium, which was one of the objects ofthe present invention, the reflectance of the as-deposited state wasdecreased to be inadequate.

[0135] In the above described preferred embodiment, the upper and lowerinterference layers were produced without applying a typical,particularly great compressive stress to the recording layer. Similar tothe above described preferred embodiment, a disc was experimentallymanufactured by forming a recording layer on typical sputteringconditions and forming upper and lower interference layers at an Ar gaspressure of 0.1 Pa and a substrate bias of 50 W. After the disc thusexperimentally manufactured was evaluated, the same result as theevaluated result of the present invention shown in table 1 was obtained.Also, when the recording film was formed by a high pressure sputteringwith a heavy rare gas and when upper and lower interference films wereformed by a low pressure bias sputtering, the same result was obtained.Although these evaluations were carried out at a linear velocity of 6m/s, the recording characteristics were good from the first time bycomplementarily using a plurality of means, as the crystallizationholding time was decreased by a high linear velocity operation.

[0136] As described above, according to this preferred embodiment, sinceit is possible to obtain a good recording characteristic from anas-deposited state, it is possible to improve the qualities of addresssignals and servo signals of a medium having an LtoH structure. Inaddition, since it is possible to remove an initial crystallizing stepfrom a process for producing a phase change medium, it is possible toachieve the reduction of the producing costs of the medium, the laborsaving of the producing process, and the space saving.

[0137] (Second Preferred Embodiment)

[0138] The second preferred embodiment of the present invention will bedescribed below. This preferred embodiment has a unique characteristicthat the distribution in grain side of crystal grains with respect tothe number of the crystal grains has a plurality of maximum values whenthe state of a phase change medium is a crystalline state.

[0139] The means necessary for carrying out this preferred embodimentand the operation thereof will be described in detail below.

[0140] As described above, the laser power and crystallizing timenecessary for crystallizing an as-deposited optical recording film areconventionally different from the laser power and crystallizing timenecessary for crystallizing an amorphous part formed by the fusedamorphous material formation (i.e., recording). The reason for this isthat the fine structure of an as-deposited amorphous state is differentfrom that of an amorphous state formed by a subsequent optical recordingoperation. That is, the cooling rate in a film deposition process duringa sputter deposition is very high (which is estimated to be 10¹² K/sec),which is ten to hundred times as higher as the cooling rate in anamorphous part forming process during a recording operation. Therefore,it is considered that the amorphous material formed in the recordinglayer in the as-deposited state has a higher randomness, i.e., a smallershort-range order, than that of the amorphous part after recording.

[0141] On the other hand, the inventor obtained a recording layercapable of immediately recording (as-deposited recording) without theneed of the initial crystallizing step, by repeating theoretical studiesand experimental manufactures. Then, after the details of the finestructure of the obtained recording layer were examined, it was foundthat the obtained recording layer had unique characteristics differentfrom those of conventional recording layers, in the distribution incrystal grain after being crystallized. That is, it was revealed that inthe optical recording medium in this preferred embodiment, thecrystalline state formed by irradiating with laser beams of an eraselevel was an aggregate of fine crystals having different crystal grainsizes, and the distribution in grain size of the fine crystals had aplurality of maximum values.

[0142] Specifically, the optical recording medium in this preferredembodiment has a phase change recording layer which reversibly changesbetween a crystalline state and an amorphous state by irradiation withlight, and is characterized in that the distribution in the number ofcrystal grains constructing the recording layer with respect to thegrain size thereof has the maximum values with respect to at least twodifferent grain sizes when the state of the phase change recording layeris the crystalline state.

[0143] That is, the recording layer is a polycrystalline substance,which has the distribution in crystal grains having peaks with respectto a large crystal grain and a small crystal grain.

[0144] In addition, the area occupied by crystal grains, which belong tothe distribution about the maximum value with respect to the small grainsize of the at least two different grain sizes, is preferably in therange of from 20% to 90% of the area of phase change recording layer.

[0145] In addition, the small grain size of the at least two differentgrain sizes is preferably in the range of from 4 nm to 20 nm, and thelarge grain size of the at least two different grain sizes is preferablygreater than 20 nm and not greater than 100 nm.

[0146] In order to achieve the as-deposited recording, when a disc isset in a recording and/or reproducing system to be operated at the samehigh linear velocity as that during actual use to be irradiated withlaser beams of an erase power level, it is required that the disc issufficiently crystallized by one irradiation so that the reflectance ofthe disc is completely a crystallization level. Specifically, the highlinear velocity herein has a value of, e.g., 6 m/sec or more. When thepickup light has a wavelength of about 630 nm to about 660 nm and an NAof about 0.55 to about 0.65, the laser power during recording has threelevels, an erase level Pe, a recording level Pw and a readout level Pr.Specifically, a power having a Pe of about 3 mW to about 6 mW, a Pw ofabout 10 mW to about 15 mW and a Pr of about 1.0 mW is selected.

[0147] When a recording is carried out, a part irradiated with laserbeams of a Pe level is crystallized, and a part irradiated with laserbeams of a Pw level is amorphous. A part which is not irradiated withlaser beams (a part other than an object track) remains being theas-deposited amorphous. With respect to the as-deposited recording, itis important that a signal level from a crystallized region (i.e., aregion between marks) after the first recording operation is notdifferent from a signal level from a crystallized region after anoverwrite is repeated two times or more. This state can be confirmed byobserving, e.g., a reproduction waveform by an oscilloscope.

[0148] When a recording film suitable for an as-deposited recording isused, it is desired that a disc is designed to be “Low-to-High”, i.e.,so that the reflectance from a crystal part is lower than thereflectance from an amorphous part, in order to facilitate the trackingand the reading of data of a header part formed by irregularitiespreviously embedded in a substrate. In the Low-to-High, a regenerativesignal level from an erase part (crystalline) is lower than that from arecording mark part (amorphous). Ideally, it is desired that a partirradiated with laser beams of a Pe level is optically completelycrystallized by only one recording operation, and a part irradiated withlaser beams of a Pw level is optically complete amorphous.

[0149] That is, in the Low-to-High, it is desired that a signal from thePe level irradiated part is sufficiently “Low”, i.e., sufficientlylowered. However, in the case of a conventional optical recordingmedium, it was confirmed that the regenerative signal level was notsufficiently lowered by the Pe irradiation once, and the regenerativesignal level tended to be gradually lowered by repeating an overwritetwo times or more. On the other hand, in the case of a recording mediumin this preferred embodiment, it was confirmed that the signal levelfrom the Pe irradiated part was sufficiently lowered by only oneirradiation with a laser beam modulated by the above described signal.That is, in the case of a recording medium in this preferred embodiment,it was found that the signal level from the Pe irradiated part was notchanged by repeating an overwrite two times or more, and sufficientlycrystallized by only one irradiation. On the other hand, in a recordingmedium in this preferred embodiment, the signal level from the Pwirradiated part was the same as that from the as-deposited region.

[0150] The optical recording medium in this preferred embodiment has afine structure wherein a part between marks (an erase level Peirradiated part) is sufficiently crystallized by only one recordingoperation from an as-deposited state so that crystal grains havingrelatively large grain sizes are surrounded by a large number of crystalgrains having relatively small grain sizes. Such a fine structure can beidentified by observation using, e.g., a TEM (Transmission ElectronMicroscope).

[0151] That is, in the TEM image obtained by the TEM observation, therecording mark part (amorphous part) is observed as a uniform regionhaving poor contrast. On the other hand, the Pe irradiated part (crystalpart) between marks is observed as crystal grains in accordance with thedegree of crystallization, and as an aggregate of fine crystal grainshaving different contrast in accordance with crystal orientation when itis sufficiently crystallized. This tendency is generally the same evenin the case of the recording films of any discs.

[0152]FIG. 4 is a schematic diagram showing an example of a TEM image ofa recording layer of a part between marks of an optical disc in thispreferred embodiment. This figure shows a state of an optical disc inthis preferred embodiment after being irradiated with laser beams of aPe level only once after a recording layer is deposited. As can be seenfrom this figure, the recording layer in this preferred embodiment hascrystal grains in all of parts between marks, and is completelycrystallized to be polycrystalline. In addition, drawing attention tothe crystal grain size, it can be seen that the recording layer in thispreferred embodiment has a fine structure wherein crystal grains havingrelatively large grain sizes are tightly surrounded by crystal grainshaving relatively small grain sizes.

[0153] On the other hand, an overwrite was repeated ten times withrespect to the optical disc in this preferred embodiment, and the TEMobservation of the recording layer of the part between marks was carriedout. As a result, the obtained TEM image was substantially the same asthat in FIG. 4. That is, in the case of the optical disc in thispreferred embodiment, it was found that the fine structure of therecording layer between marks was completely crystallized to bedetermined by the first irradiation with laser beams of an erase levelfrom the as-deposited state, and was not changed by the subsequentoverwrite.

[0154] A method for preparing a sample for the TEM observation will bedescribed below. First, after a recording operation is carried out in arecording medium by the above described method, a metal reflective filmand a substrate are removed, and the resulting medium is put on a metalmesh to be used as a sample to be observed. The removal of the substratemay be carried out using an organic solvent. However, it is required toprevent heat from being applied to the sample when the sample isprepared. Therefore, even if the ion milling method is used, it isrequired to fully carefully prevent the sample from being heated to 150°C. or higher.

[0155] By analyzing the example of the TEM image shown in FIG. 4 usingan image processing, the distribution in grain size can bequantitatively derived.

[0156]FIG. 5 is a graph showing the distribution in crystal grain sizein a part between marks of an optical disc in this preferred embodiment.This figure is obtained by plotting the distribution in appearancefrequency of large crystal grains having a grain size of 20 nm or more,which is shown by A in the example of the TEM image of FIG. 4. In theexample of FIG. 4, the proportion of the area occupied by crystal grainsbelonging to the distribution about the maximum value in the small grainsize of the at least two different grain sizes is about 75%.Furthermore, this distribution was obtained in a region about 8.5 μmsquare extracted from the TEM image at random. The “grain size” isherein defined to be the average of the longest diameter and shortestdiameter of one crystal grain, which are measured.

[0157]FIG. 5(a) shows a case where only one recording operation wascarried out from the as-deposited state, and FIG. 5(b) shows a casewhere an overwrite was repeated ten times. It can be seen from FIG. 5that large crystal grains are distributed in the range of from about 20nm to about 100 nm. In addition, the distribution in crystal grain sizeis not substantially different even if an overwrite is repeated tentimes as shown in FIG. 5(b). The average of diameters of crystal grainsbelonging to the large crystal grains was 50.5 nm in FIG. 5(a) and 60.8nm in FIG. 5(b). From the above results, it can be seen that the opticalrecording medium in this preferred embodiment is sufficientlycrystallized by carrying out only one recording operation from theas-deposited state.

[0158] The grain size of the large crystal grains shown by A in FIG. 4tends to depend on various conditions, such as the composition of therecording film and the producing process thereof. However, after theinventor carried out comparative studies with respect to various phasechange recording media, it was found that the grain size of the largecrystal grains was in the range of from about 20 nm to about 100 nm.

[0159] In addition, as can be seen from FIG. 4, there are a large numberof fine crystal grains around large crystal grains shown by A.Similarly, the distribution in grain size was measured with respect tothe fine crystal grains.

[0160]FIG. 6 is a graph showing the distribution in grain size of finecrystal grains. This figure is obtained by plotting the distribution inappearance frequency of small crystal grains having a grain size of 20nm shown by B in the example of the TEM image of FIG. 4. Thisdistribution is obtained by making a graph of an example of thedistribution in appearance frequency of about 300 crystal grains havinga size of 20 nm or less observed in a region about 2 μm square which isextracted from a TEM image of a sample at random, the sample beingrecorded only once from the as-deposited state, and the TEM image of thesample being observed.

[0161] It can be seen from FIG. 6 that the grain sizes of small crystalgrains are distributed in the range of from about 4 nm to about 20 nm.The grain size corresponding to the maximum value of the distributionwas about 7 to 8 nm. In addition, this distribution was notsubstantially changed even if an overwrite was repeated ten times.

[0162] On the other hand, a conventional optical disc will be describedas follows.

[0163] That is, even if a conventional optical disc irradiated with alaser beam of a Pe level once, it is not crystallized, so that crystalgrains are not observed. However, if an overwrite is repeated,crystallization proceeds, so that relatively large crystal grains areobserved in the Pe irradiated part. If the distribution in grain size isanalyzed, although the grain sizes are different in accordance with thecomposition of the recording film, the grain sizes are distributed in acertain range about a certain mean so that the distribution does nothave a plurality of maximum values. For example, in the case ofGe₂Sb₂Te₅, the grain sizes are distributed in the range of from 70 nm to150 nm, so that the crystallized part is filled with crystal grainshaving such a size. That is, the recording layer is filled with largecrystal grains, and the distribution in grain size has a single maximumvalue.

[0164] Even if the conventional optical disc is initialized by aninitializing system, the same result is obtained. That is, it was foundthat the crystallized part was filled with relatively large crystalgrains having a grain size of about 70 to 150 nm.

[0165] As described above while comparing with the conventional opticaldisc, the optical disc in this preferred embodiment is characterized inthat crystallization occurs sufficiently if irradiation with laser beamsof an erase level is carried out only once, and the distribution ingrain size has two maximum values in large crystal grains and finecrystal grains.

[0166] It is considered that the unique fine structure of the recordingfilm in this preferred embodiment appears by the following mechanism.That is, it is well known that crystallization occurs in two stages, anucleation stage and its grain growth stage. Then, in the recordinglayer of the optical disc in this preferred embodiment, there arealready a large number of very fine regular structures capable of beingan initial nucleus for crystallization even in the as-deposited state.Such a fine regular structure has a fine crystal nucleus having a sizeof about 0.5 to 4 nm.

[0167] When temperature rises to about the crystallizing temperature byirradiation with laser beams, the fine regular structures grow, so thatsmall crystal grains having a size of about 4 nm to about 20 nm areproduced about the respective fine regular structures. In this state, alarge number of such small crystal grains are produced in the recordinglayer. These crystal grains form fine crystal grains shown by B in FIG.4. This crystal grain size is an optically important value. When thecrystal grain size is this value or less, the complex index ofrefraction approaches that of an amorphous material. If the grain sizeis 4 nm or more, the complex index of refraction is equal to that of acrystal. In addition, since the crystal grains are small, time necessaryfor the above described fine structure to grow to crystal grains havinga size of about 4 to 20 nm may be short. Therefore, even at a highlinear velocity, the state of the as-deposited disc can be changed to astate of an optically crystallized level by irradiating the disc withlaser beams of an erase level.

[0168] On the other hand, after such an initial nucleation process, acrystal grain growing process occurs. In the case of the opticalrecording medium in this preferred embodiment, a large number of nucleiare already produced in an initial stage of irradiation with laserbeams. Therefore, most of crystal nuclei can not further grow, and onlya small number of crystal grains, which has a relatively small nucleusdensity around the crystal grains and which is held in a crystal growthtemperature zone for a long time, become large crystal grains shown by Ain FIG. 4. Furthermore, of course, such a crystal having a large grainsize has optically a refractive index of a crystal.

[0169] The recording film in this preferred embodiment has the same finestructure even if it is initialized by a conventional initializingsystem. That is, bulky crystal grains having a size of 20 to 100 nm aresurrounded by fine crystal grains having a size of about 4 to 20 nm.Also in this case, the grain sizes of most of crystal grains are verysmall, so that the crystallizing time to crystallize an amorphousmaterial may be short. That is, the recording film is more suitable fora high linear velocity, and has an improved erasing rate.

[0170] In addition, the optical recording medium in this preferredembodiment is characterized in that the percentage of the area ofcrystal grains belonging to small grain sizes to that of all of thecrystal grains is in the range of from 20% to 90%. Moreover, It wasfound by the inventor's study that it was possible to more stably andsurely carry out crystallization when the percentage of the area is inthe range of from 40% to 80%. That is, when crystallization is carriedout to exhibit such a distribution in grain size, the recording film hasthe best balance of crystal nucleation and crystal growth and canaccomplish a high erasing rate. In addition, the recording film issuitable for the as-deposited recording.

[0171] As a method for producing an optical recording medium having aunique construction in this preferred embodiment, there is a methodusing krypton (Kr) and/or xenon (Xe) as a sputter gas as describedabove.

[0172] The reason why the cooling effects of Kr and Xe are superior tothose of sputter particles, such as germanium (Ge), antimony (Sb) andtellurium (Te), will be described in more detail below. That is, in thedeposition using the sputter method, sputter particles emitted from atarget collide with atmospheric gas particles before the sputterparticles reach a substrate. The kinetic energy which is lost at thistime depends on the mass of an object to collide with the sputterparticles. When approximation is carried out using a rigid bodycollision model, incident particles have the same mass as that ofcounter particles colliding with the incident particles. Assuming thatthe counter particles stand still, all of the kinetic energies of theincident particles move to the counter particles in the case of ahead-on collision. In the case of a collision other than the head-oncollision, the kinetic energy moves in a ratio corresponding to animpact parameter. If the kinetic energy is integrated over all of theimpact parameters, half of the kinetic energy of incident particlesmoves to counter particles on the average when the incident particlescollide with the same kind of the counter particles once. If a rigidbody collision model is assumed, and assuming that the mass of acolliding particle is m1 and the mass of a collided particle is m2, theproportion of lost energy is expressed by 2m1·m2/(m1+m2)².

[0173] The dominant mass numbers of Ge, Sb and Te atoms are 73, 122 and123, respectively. For example, when Sb collides with Ar (mass number:40), only 37% of the energy of Sb moves, whereas when Sb collides withXe, 50% of the energy of Sb moves. When one particle collides with otherparticles a plurality of times, there is a conspicuous differenceparticularly due to the kind of gas. In general, as the mass numbers oftwo colliding particles are close to each other, the kinetic energy isefficiently lost. Therefore, if sputter particles collide with Kr or Xe,the energies thereof are sufficiently lowered, i.e., temperature islowered, and then, the sputter particles reach the substrate. Therefore,while the temperature of the sputter particles on the substrate becomesroom temperature, the cooling rate decreases, so that the randomness ofthe film decreases. As a result, it is possible to obtain an amorphousmaterial having a randomness approximate to that of an amorphous partafter an optical recording operation. Although the degree of the lostkinetic energy of the sputter particle depends on a collision rate, thecollision rate ν is expressed by ν=1/n σ (σ is a collision crosssection, and n=p/k_(B)T is a gas particle density), so that thecollision rate can be suitably adjusted by an atmospheric gas pressure.

[0174] Referring to concrete examples, this preferred embodiment will bedescribed in more detail below.

FIRST EXAMPLE

[0175]FIG. 7 is a conceptual sectional view of the first preferredembodiment of a phase change recording medium according to the presentinvention.

[0176] In this figure, reference number 201 denotes a substrate, 202denoting a first interference layer, 203 denoting a recording layer, 204denoting a second interference layer, and 205 denoting a reflectivelayer. This medium was prepared by the following method. First, anoptical disc substrate 201 of a polycarbonate having grooves having awidth of 0.6 μm is mounted in a substrate holder of a multi-chambersputtering system. Then, a first interference layer 202 having athickness of 80 nm is deposited by the RF sputtering method in asputtering chamber having a ZnS—SiO₂ composite target. Then, a recordinglayer 203 having a thickness of 20 nm is deposited by the DC sputteringmethod in a sputtering chamber having GeSbTe target. Subsequently, asecond interference layer 204 having a thickness of 30 nm is depositedby the RF sputtering method in a sputtering chamber having a ZnS—SiO₂composite target. Finally, a reflective layer 205 having a thickness of50 nm is deposited by the DC sputtering method in a sputtering chamberhaving an Al target.

[0177] As a sputtering gas, pure Ar was used for depositing layers otherthan the recording layer 203, and a mixed gas of Ar and Kr was used fordepositing the recording layer 203. The composition of the gas wasAr:Kr=1:10, and the total pressure was 4.0 Pa. As a vacuum gage formeasuring a gas pressure, a diaphragm gage suitable for the measurementof the total pressure was used. The probe of the diaphragm gage wasprovided at a location wherein the probe was not under the influence ofthe position distribution in gas pressure, not at a location near a gasinlet. To the disc substrate ejected after the thin-film deposition iscompleted, a dummy substrate (thickness: 0.6 mm) was bonded by an UVresin to form a medium to be evaluated.

[0178] This recording medium was evaluated on at a constant linearvelocity. While the state of the recording medium remained being theas-deposited state, a 3T signal having a linear velocity of 8.2 m/secand a clock frequency of 116.45 MHz was recorded on an as-depositeddisc. Thus, a CNR of 52.5 dB was obtained. In addition, when randomsignals of 3T to 11T were recorded on different tracks in theas-deposited state, a jitter value of 8.2% was obtained. In this track,an overwrite was repeated ten times, so that the jitter value waschanged to 8.4%. Moreover, after an overwrite was repeated hundred timesand thousand times, the jitter values were measured. The measured jittervalues were in the range of from 8 to 9%.

[0179] Thus, it was possible to obtain a good jitter and repetitioncharacteristic by recording directly in the as-deposited track.

[0180] Then, the crystalline state of the crystal part formed in therecording film in this preferred embodiment was examined by the TEM.First, the as-deposited track was irradiated only once with a laserbeam, which was modulated by an 11T signal at the above described linearvelocity and recording clock frequency, to form a mark row, and aregenerative signal waveform was observed. In this experiment, after therelationship between laser powers Pe of an erase level and regenerativesignal levels from a location between marks was measured, theregenerative signal level was minimum at Pe=4.5 mW. The regenerativesignal level is reflected in the reflectance of a track which is readout by laser beams. Therefore, the minimum of the regenerative signallevel exhibits the optimum crystallizing condition. Thus, the optimumerase power was found.

[0181] Then, a sample for the TEM observation was formed. That is, an11T signal was written in another as-deposited track at Pe=4.5 mW andPw=12 mW by only one rotation of a disc. This was carried out in aplurality of tracks so as to facilitate observation as the TEM sample.This part will be hereinafter referred to as an “as-deposited recordingpart”. In addition, a sample overwritten in different tracks a pluralityof times at the same recording condition was prepared for comparison.Then, the Al reflective layer 205 and the substrate 201 were removed,and the residue was put on a metal mesh to be used as a sample to beobserved. The Al reflective layer 205 was removed by a method forputting mesh-like scratches on the film to apply a tape thereon by anadhesive to peel off the layer. In addition, the substrate 201 wasdissolved in an organic solvent to be removed. According to thesemethods, there is no heating process for changing the crystalline state.

[0182] First, a TEM bright field image of a part between marks of theas-deposited recording part was observed. From this result, the erasepart was the same polycrystalline substance as that in the example ofFIG. 4, and microcrystal and amorphous parts, which were not able to beresolved by the TEM, were not particularly observed. The respectivecrystal grains were observed by their crystal orientation as anaggregate of microcrystals having different contrasts.

[0183] On the basis of the TEM image, the grain size of the crystalgrains was measured, and the appearance frequency thereof was examined.It was found by the analysis that two kinds of crystal grains, one kindof which belongs to fine crystal grains having a size of 4 to 20 nmabout 8 nm and the other kind of which belongs to large crystal grainshaving a size of 20 to 100 nm, can be clearly distinguished from eachother in both regions of the Pe irradiated region of the as-depositedrecording part and the Pe irradiated region after the repeatedoverwrite. After the distribution in grain size of the large crystalgrains and fine crystal grains was analyzed, the same graphs as FIGS. 5and 6 were obtained.

[0184] Thus, it was revealed that the recording film in this preferredembodiment suitable for the as-deposited recording is characterized inthat it is polycrystallized by irradiation with a laser beam of an eraselevel only once and that the distribution in grain size of the formedcrystal grains comprises relatively large crystal grains distributed inthe range of from 20 to 100 nm, and crystal grains which are filledaround the relatively large crystal grains and which are distributed inthe range of from 4 to 20 nm. In other words, it was revealed that thedistribution has a plurality of maximum values when the appearancefrequency of grain sizes is a function with respect to grain sizes. Inaddition, it was revealed that the distribution in grain size of thecrystal part also has a plurality of maximum values even if an overwriteis repeated a plurality of times.

[0185] While the recording film has been formed of GeSbTe in thisexample, the recording film may be formed of InSbTe, AgInSbTe, AuInSbTeor any one of these material systems containing additional elements, orother phase change recording films may be used to obtain the sameadvantages as those in this example. In addition, the recording film inthis example is not only applied to a repeatable medium, but it may alsobe applied to a once writable or rewritable type recording medium, suchas a so-called CD-R or CD-RW to obtain the same advantages as those inthis example.

COMPARATIVE EXAMPLE

[0186] For comparison with the above described example, an optical dischaving a recording film 203 formed by a typical deposition process wasprepared, and the same experiment was carried out. The construction andthickness of the films of the disc were the same as those in the firstexample. Films other than the recording film were deposited by the samemethod as that in the first example. As a sputter gas for the recordingfilm, Ar gas was used, and the gas pressure was adjusted to be 1.0 Pa.

[0187] With respect to this disc, the as-deposited track was irradiatedonce with a beam, which was modulated by an 11T signal at the samelinear velocity and recording clock frequency as those in the firstexample, to form a mark row to observe a regenerative signal waveform.As a result, the signal level from the Pe irradiation part was notsufficiently lowered by the Pe irradiation once. Moreover, the Pe waschanged to carry out an experiment. When the Pe was less than 2.5 mW,there was no change. When the Pe was 2.5 mW or more and less than 6 mW,the reflectance was slightly lowered. However, the reflectance did notreach a saturated level, and was close to the signal level from theas-deposited part. In addition, when the power was 6 mW or more, thereflectance was not lowered. The reason for this is considered that thepower is too high, so that the recording layer is partially molten andchanged to be amorphous again when being cooled.

[0188] When an overwrite was carried out twice or more at a Pe of 2.5 mWor more and less than 6 mW, the reflectance was gradually lowered whilethe overwrite was repeated. When the overwrite was repeated five timesor more, the signal level from the Pe irradiated part reached thesaturation level. Therefore, assuming that the optimum Pe level duringactual use was a power, at which a signal level from a part betweenmarks after an overwrite was repeated ten times was minimum, the optimumPe level was 4.0 mW.

[0189] Then, a sample for the TEM observation was formed. That is, an11T signal was written in different as-deposited tracks at Pe=4.5 mW andPw=12 mW by only one rotation of a disc. This was carried out in aplurality of tracks so as to facilitate observation as the TEM sample.This part will be hereinafter referred to as an “as-deposited recordingpart”. In addition, a part overwritten in different tracks a pluralityof times at the same recording condition was prepared for comparison.

[0190] Then, the Al reflective layer and the substrate were removed, andthe residue was put on a metal mesh to be used as a sample to beobserved. First, the TEM image of a part, at which a recording wascarried out only once, was observed. As a result, it was found that whenthe as-deposited recording was carried out, the Pe irradiated part had auniform contrast wherein crystal grains were not clearly identified, sothat the Pe irradiated part remained being amorphous. This is supportedby the fact that only a halo-like pattern was observed as an electronbeam diffraction pattern of the Pe irradiated part. In addition, therecording mark part (amorphous) was observed as a uniform region havinga poor contrast.

[0191] On the other hand, when an overwrite was repeated in the samedisc, crystal grains were clearly identified in the Pe irradiated part.After the distribution in grain size of the Pe irradiated part wasanalyzed, it was revealed that most of grain sizes were in the range offrom 70 to 150 nm.

[0192]FIG. 8 is a graph wherein the appearance frequencies of measuredindividual crystal sizes are plotted. That is, in this figure, theappearance frequencies of the grain sizes of all of the crystal grainsare plotted from a region about 4.5 μm optionally obtained in a TEMphotograph. As can be seen from FIG. 8, in a conventional optical disc,the distribution in grains size had a single maximum, and the maximumvalue thereof was about 110 nm. That is, it was found that the recordinglayer was filled with relatively large crystal grains.

[0193] In addition, after a track recorded twice for comparison wasexamined by the same TEM observation, it was found that crystallizationdid not proceed, so that most of regions were amorphous although crystalgrains were partially observed.

SECOND EXAMPLE

[0194] The second example of this preferred embodiment of a phase changeoptical recording medium according to the present invention will bedescribed below.

[0195] In this example, the phase change optical recording medium hasthe same cross section as that of FIG. 7. However, the thickness of eachof layers was designed so as to be suitable for a so-called High-to Lowconstruction which has a high reflectance when the state of a recordingfilm 203 was a crystalline state.

[0196] That is, while the material of each of the layers was the same asthe first example of this preferred embodiment, the thickness of a firstinterference layer 202, the thickness of a recording layer 203, thethickness of a second interference layer 204, and the thickness of areflective layer 205 were 160 nm, 20 nm, 5 nm and 150 nm, respectively.The procedure for producing a recording medium was the same as that ofthe first example of this preferred embodiment.

[0197] Then, the recording medium thus produced was loaded in aninitializing system to initialize the whole surface thereof. The laserbeam of the used system was an elliptical beam having a size of 95 μm×1μm. The initializing conditions included a linear velocity of 2 m/sec, afeed pitch of 12 μm and a laser power of 300 mW.

[0198] Then, the recording medium thus initialized was evaluated at aconstant linear velocity. First, when a 3T signal having a linearvelocity of 8.2 m/s and a clock frequency of 116.45 MHz was recorded, aCNR of 51.2 dB was obtained. Moreover, when an 11T signal was recordedon another initialized part of the disc once, a CNR of 56.5 dB, whichwas a good value as the CNR of the 11T signal, was obtained.

[0199] Then, when the 11T signal was overwritten by the 3T signal, theintensities of these signals were as follows.

[0200] 11T carrier level: −47.2 dBm

[0201] 3T carrier level: −12.5 dBm

[0202] effective erasing ratio: 34.7 dB

[0203] The “effective erasing ratio” is defined as a difference betweenthe signal strength of the written 11T signal and the signal strength ofthe overwritten 3T signal.

[0204] Then, when random signals from 3T to 11T were recorded on anotherinitialized track, a jitter value of 9.5% was obtained. When anoverwrite was repeated in this track, the jitter value was changed inthe range of from 8% to 9%, so that it was confirmed that a very goodjitter characteristic was obtained.

[0205] The reason why the good repetition characteristic was thusobtained is that a recorded amorphous mark was sufficiently erased bythe overwrite as can be seen from the fact that the erasing rate wasgood, 34.7 dB.

[0206] Then, the crystalline state of the recording film was examined bythe TEM. A part, which was only initialized and which was not recorded,was observed. A track, in which an overwrite was carried out a pluralityof times, was also formed in the initialized part for comparison.

[0207] First, a sample was prepared by the same method as that of thefirst example, and the TEM bright field image of the initialized partwas observed. The result was the same as the example of FIG. 4, and theinitialized part was clearly polycrystalline. A fine crystal oramorphous part, which was not able to be dissolved by the TEM, was notdetected. The respective crystal grains were observed as an aggregate offine crystal grains having different contrast in accordance with theircrystal orientation.

[0208] The grain sizes of the crystal grains were measured on the basisof the observed TEM image, and the appearance frequencies thereof wereexamined. First, the bright image of a region 8 μm square was picked up,and the grain sizes of crystal grains belonging to crystal grains of alarge grain size were measured. Specifically, the longest diameter andthe shortest diameter of the respective crystal grains were measured,and the average thereof was obtained to be recorded. The diameters ofthe respective crystal grains were distributed in the range of from 20to 100 nm, and its average was 60 nm.

[0209] Then, when crystal grains of other parts belonging to crystalgrains of a small grain size were observed at a high power, any partswere aggregates of relatively small grains having a grain size of 4 to20 nm.

[0210] In addition, when a crystal part formed by repeating theoverwrite was observed, it was revealed that the crystal part wassimilarly divided into large crystal grains and parts surrounded by andfilled with fine crystal grains.

[0211] Furthermore, while the recording film 203 has also been formed ofGeSbTe in this example, the recording film may be formed of InSbTe,AgInSbTe, AuInSbTe or any one of material systems containing additionalelements to obtain the same advantages as those in this example. Inaddition, the recording film in this example is not only applied to arepeatable medium, but it may also be applied to a once writable orrewritable type recording medium, such as a so-called CD-R or CD-RW toobtain the same advantages as those in this example.

[0212] While this preferred embodiment of the present invention has beendescribed in the terms of the examples, this preferred embodiment shouldnot be limited to these examples.

[0213] For example, in the above described example, a four-layerstructure of ZnS—SiO₂/GeSbTe/ZnS—SiO₂/Al deposited on a substrate bysputtering is illustrated. However, a five-layer structure wherein an Ausemitransparent film is provided in the four-layer structure may beused. In addition, the material and thickness of each of the layers, andthe method and conditions for depositing films other than the recordingfilm should not be limited, except for the conditions for sputtering andinitializing the recording layer which are important in this preferredembodiment.

[0214] For example, in the case of a five-layer structure, thesemitransparent layer may also be formed of copper (Cu), silicon (Si) ora film having a structure wherein fine metal particles are dispersed ina dielectric matrix, in place of Au.

[0215] In addition, in place of the semitransparent film of thefive-layer structure, a laminated film of two or more layers of two ormore kinds of transparent film materials having different refractiveindexes may be used. For example, if a film formed by sequentiallylaminating a ZnS film or a mixed film of ZnS and SiO₂, an SiO₂ film, anda ZnS film or a mixed film of ZnS and SiO₂, which have an appropriatelyselected thickness, is used, it is possible to provide a medium moresuitable for a high density recording.

[0216] In addition, the material of the interference layer may besuitably selected from dielectric film materials, such as Ta₂O₅, Si₃N₄,SiO₂, Al₂O₃ and AlN, in place of ZnS—SiO₂, and the material of therecording layer may be suitably selected from chalcogen film materials,such as InSbTe, AgInSbTe and GeTeSe, in place of GeSbTe. The material ofthe reflective layer may be suitably selected from Al alloy filmmaterials, such as AlMo, AlCr and AlTi, in place of Al.

[0217] Moreover, while the optical disc has been described as an exampleof an optical recording medium in the above described example, thispreferred embodiment should not be limited thereto. For example, thispreferred embodiment may be applied to any one of various opticalrecording media, such as an optical recording card, to obtain the sameadvantages as those of this preferred embodiment.

[0218] This preferred embodiment is applied in the above described formto have the undermentioned advantages.

[0219] First, according to this preferred embodiment, it is possible toprovide an optical recording medium, which can be used immediately in anas-deposited state without passing through an additional process forinitial-crystallizing a recording film and which has excellent recordingcharacteristics and high repeatable overwrite characteristics.

[0220] As a result, it is possible to reduce equipment investments,operation costs and time necessary for initial crystallization, and itis possible to provide an inexpensive, high performance opticalrecording medium.

[0221] For example, in the case of a conventional optical recordingmedium, a tact time per disc in a deposition step is several seconds,whereas a tact time in an initialization step is several minutes, whichserve as a great bottle neck for production. On the other hand,according to this preferred embodiment, it is possible to reduce such aninitialization step, so that it is possible to greatly improveproduction throughput.

[0222] Third Preferred Embodiment

[0223] The third preferred embodiment of the present invention will bedescribed below.

[0224] In this preferred embodiment, there is provided a phase changerecording medium which has a recording layer, wherein a phase changebetween a crystal phase and an amorphous phase is caused by irradiationwith light from the outside, as means for accomplishing the abovedescribed object, and which is characterized in that the recording layercontains at least one of krypton and xenon.

[0225] The total percentage content of krypton and xenon in therecording layer is preferably in the range of from 0.2 to 10 at. %(atomic percent).

[0226] This allows the amorphous state of the recording layerimmediately after sputtering deposition, i.e., in the as-depositedstate, to approach the amorphous state after an optical recordingoperation to allow a recording reproducing operation without the need ofany initial crystallizing steps.

[0227] The inventor found that it was possible to obtain the optimumcharacteristic of a recording film of an optical recording medium whenthe film contains krypton (Kr) and/or xenon (Xe), particularly when theatomic fraction of KR and/or Xe is in the range of from 0.2 to 10%, as amethod for depositing an amorphous material having a favorablerandomness in the as-deposited state with a good controllability.

[0228] When an amorphous mark is recorded in the as-deposited state ofthe phase change optical disc thus formed, a unique construction isformed. That is, a region between marks is crystallized, so thatamorphous marks are scattered in a narrower crystallized band than thewidth of a track. The width of a mark is narrower than or equal to thewidth of the crystallized band. When it is equal to the width of thecrystallized band, the amorphous parts are connected in the form of anetwork, and the crystallized region between marks exists therein. Whenthe linear velocity is not high and when the end portion of theamorphous mark is crystallized, a slightly narrower crystal band thanthe width of a track is formed, and a slightly narrower amorphous markthan the width of the band exists in the band. The region between thecrystal bands holds the amorphous state immediately after the sputteringformation.

[0229] That is, in this preferred embodiment, there is provided a phasechange optical disc wherein the reflectance (Rc) of the crystal part isadjusted to be lower than the reflectance (Ra) of the amorphous partviewed from an optical detection side and wherein a recording state hasan amorphous mark row having a width, which is narrower than or equal tothe width of a crystal band than the width of a track, in the crystalband, the state of the phase change recording layer of an address partbeing an amorphous state in the phase change optical disc wherein the Rcis adjusted to be lower than the Ra viewed from the optical detectionpart.

[0230] In the state wherein the width of the crystal band is equal tothe width of the amorphous mark, the end portion of the amorphous markis connected to the amorphous band between the crystal bands. In thiscase, although it is difficult to define the width of the mark, it isdefined in this preferred embodiment that the width of the markcorresponds to that of the crystal band when the width of the mark isconnected to the amorphous band.

[0231] In general, a groove for a tracking guide is provided on anoptical disc substrate, and a medium film is deposited thereon, so thatthe region between tracks is a groove stepped part. In this preferredembodiment, an optical disc is characterized in that the state of atleast part of the groove stepped part is an amorphous state. Theamorphous state between tracks may be formed after a recording operationis carried out in an unrecorded disc at least once, and may be changedto a crystalline state after an overwrite is carried out a plurality oftimes. It can be determined by, e.g., the electron beam diffractionmethod for irradiating with stopped electron beams, whether the state ofthe region between tracks is an amorphous state or a crystalline state.

[0232] Referring to the accompanying drawings, an example of thispreferred embodiment will be described below.

[0233]FIG. 9 is a schematic sectional view showing an example of a phasechange optical disc in this preferred embodiment. In this figure,reference number 301 denotes a substrate, 302 denoting a firstinterference layer, 303 denoting a recording layer, 304 denoting asecond interference layer and 305 denoting a reflective layer. Theoptical sick of FIG. 9 can be produced by, e.g., the following method.First, an optical disc substrate 301 of a polycarbonate having grooveshaving a width of 0.6 μm is mounted in a substrate holder of amulti-chamber sputtering system.

[0234]FIG. 10 is a schematic diagram showing a principal part of asputtering system for use in this preferred embodiment. In this figure,reference number 311 denotes a sputtering chamber, 312 denoting a discsubstrate, 313 denoting a sputtering target, 314 denoting a sputteringsource, 315 denoting a sputtering power supply, 316 denoting a gasintroducing system, 317 denoting an exhaust system, and 318 denoting asubstrate surface migration control system. The sputtering chamber ofFIG. 10 was used for forming the recording layer 303. Other layers weredeposited in other sputtering chambers having similar constructionsusing their sputtering methods and conditions.

[0235] First, the substrate 301 is transported to a sputtering chamberhaving a ZnS—SiO₂ composite target to deposit a first interference layer302 having a thickness of about 80 nm by the RF sputtering method. Then,the substrate 301 is transported to a sputtering chamber having a GeSbTetarget to deposit a recording layer 303 having a thickness of about 20nm by the DC sputtering method. Subsequently, the substrate 301 istransported to the sputtering chamber having the ZnS—SiO₂ compositetarget again to deposit a second interference layer 304 having athickness of about 30 nm by the RF sputtering method. Finally, thesubstrate 301 is transported to a sputtering chamber having an Al targetto deposit a reflective layer 305 having a thickness of about 50 nm bythe DC sputtering method.

[0236] When the recording layer 303 was deposited, a mixed gas of Ar andXe was used as a sputtering gas. The composition of the gas wasAr:Xe=1:4, and the gas pressure was 2.0 Pa.

[0237] After a sample deposited on the same conditions as those of thedeposition of the recording layer 303 was analyzed by a Rutherford backscattering (RBS), it was revealed that the composition of contained Xewas 2.5 atomic %.

[0238] This optical disc was evaluated at a constant linear velocitywithout carrying out the initial crystallizing step. After a 3T signalhaving a clock frequency of 116.45 MHz was recorded on an as-depositeddisc at a linear velocity of 8.2 m/sec in accordance with the DVDstandard, a CNR value of 52.5 dB was obtained. After random signals from3T to 11T were recorded on another as-deposited track, a jitter value of8.5% was obtained. When an overwrite was repeated in this track, thejitter value gradually decreased, and it was 8.4% when the overwrite wasrepeated ten times. Moreover, when the overwrite was repeated hundredtimes and thousand times, both of the measured jitter values were in therange of from 8 to 9%.

[0239] Thus, according to this preferred embodiment, it is not requiredto carry out the initial crystallization, and it is possible to obtaingood CNR and repetition characteristics even if a recording is carriedout directly in an as-deposited track.

[0240] The inventor studied the details of the relationship between theamount of a rare gas contained in the recording layer 303 and thecharacteristic of the disc.

[0241]FIG. 11 is a graph showing the relationship between the amount ofKr contained in a recording layer and a 3T jitter characteristic. Thecontent of Kr was measured by the RBS. In addition, the content ratio ofKr in the recording layer depends on the pressure of Kr gas, thethin-film deposition rate and the substrate bias when the recordinglayer is deposited. That is, as the gas pressure increases, or as thethin-film deposition rate decreases, there is a tendency for the amountof Kr incorporated into the film to increase. In addition, with respectto the substrate bias power, the content ratio of Kr was highest when anappropriate bias was applied. In general, it is considered that, as thecontent of Kr increases, the randomness of the recording layer decreasesto approach an amorphous state after an optical recording operation.

[0242] In FIG. 11, black plotted points denote jitter amounts when a 3Tmark row is recorded once without carrying out initialization after athin-film deposition, and white plotted points denote jitter amountswhen a 3T mark row is recorded after the recording of a 3T mark row andthe DC erase are repeated hundred times. The jitter amount on the axisof ordinates of the graph is expressed by percentage with respect to thewidth of a window.

[0243] The standard of an allowable amount of 3T jitter including apractical operating margin as a drive is 8%. When the jitter is lessthan this standard, a low BER (bit error rate) operation having a highreliability is ensured, and when the jitter is not less than thestandard, there is no reliability. As can be seen from FIG. 11, theinitial recording jitter (black circle) is higher than 8% when thecontent ratio of Kr is less than 0.2 at % (atomic percent), so that itis difficult to ensure the low BER operation. On the other hand, thejitter exceeded 8% again when the content ratio of Kr was 20 at % ormore. That is, it was found that it was possible to obtain a good jittercharacteristic in a content therebetween.

[0244] The reason why the jitter is high when the content ratio of Kr isless than 0.2 at % is that the relaxation of the cooling rate of therecording layer during the thin-film deposition is insufficient in thisrange to lack the formation of the short-range order state, so that itis difficult to carry out crystallization from a non-initializedamorphous state. On the other hand, the rapid increase of the jitter ata Kr content ratio of less than 20 at % is based on the decrease of theCNR which is based on the decrease of the amplitude of a signal. It isconsidered that this is caused by the fact that the change of theoptical characteristic between a crystal and an amorphous materialdecreases in a recording layer containing an excessive amount of Kr of20 at % or more.

[0245] The jitter (white circle) after repeated recording is held to bea sufficient low value from a conventional recording layer containing noKr (the white circuit at the left end in FIG. 11; “0” in 0 of an arrowextending to the left means that Kr is contained), to a film having a Krcontent ratio of 10 at %, and rapidly increases when the Kr contentratio is 10 at % or more. Because in the case of a film containing 10 at% Kr or more, atomic diffusion proceeds by repeating heating and coolingcycles, so that the state of the film becomes a phase separation state.Therefore, it can be seen that the Kr content ratio, in which a lowjitter operation can be carried out after a recording operation isrepeated after carrying out the initial recording without carrying outthe initialization step, is in the range of from 0.2 to 10 at %. Afterthe same experiment was also carried out with respect to Xe, the contentratio of Xe was in the range of from 0.2 to 12 at %, which was aslightly wide range, so that a low jitter characteristic was obtainedover the repeated recording from the initial recording.

[0246] That is, according to this preferred embodiment, it is possibleto promote the short-range order state in the as-deposited state bycarrying out a sputtering deposition using a gas, such as Kr and Xe, andselecting deposition conditions so that the composition of the gas is ina predetermined range of composition, and it is possible to obtain avery good light writing characteristic without carrying out the initialcrystallizing step.

[0247] Furthermore, while GeSbTe has been used as the material of therecording layer in this preferred embodiment, InSbTe or any one ofmaterial systems containing additional elements added thereto may beused to obtain the same advantages as those in this preferredembodiment. In addition, this preferred embodiment is not only appliedto a repeatable medium, but it may also be applied to a once writable orrewritable type recording medium, such as a so-called CD-R (compactdisc-recordable) or CD-RW to obtain the same advantages as those in thispreferred embodiment.

[0248] An example of this preferred embodiment, wherein the state of atrack after recording was examined, will be described below.

[0249]FIG. 12 is a sectional view showing the schematic construction ofan optical disc in this preferred embodiment. That is, the optical discin this preferred embodiment comprises a semitransparent film 322, alower interference film 323, a recording layer 324, an upperinterference film 325 and a reflective layer 326, which are laminated ona substrate 321 in that order. The substrate 321 is a pre-formattedpolycarbonate substrate having a diameter of 120 mm and a thickness of0.6 mm, and comprises an address part of pre-pit strings and a datapart, in which pre-grooves are formed. A method for producing thesubstrate 321 comprises the typical steps of: mastering a master disc,preparing a stamper by plating, and injection-forming a polycarbonateresin on the stamper. In this preferred embodiment, the track width ofeach of grooves G and lands L was set to be 0.74 μm, which was the sameas that of the first generation DVD-RAN digital versatile disc-randomaccess memory) standard, and the depth of each of the grooves G was setto be 70 nm, which was the same as that of the stroke cancelspecification.

[0250] The respective layers formed on the substrate 321 include the Ausemitransparent layer 322 having a thickness of 10 nm, the lowerZnS—SiO₂ interference layer 323 having a thickness of 85 nm, the GeSbTerecording layer 324 having a thickness of 10 nm, the upper ZnS—SiO₂interference layer 325 having a thickness of 30 nm and the AlMoreflective layer 326 having a thickness of 100 nm, which are arranged inthat order from the substrate surface. The composition of each of theupper and lower ZnS—SiO₂ interference films was a standard compositioncontaining 20 at. % SiO₂, and the composition of the GeSbTe recordinglayer was a standard composition wherein Ge:Sb:Te=2:2:5. The opticaldesign values of an optical disc having the above described structureinclude an Rc (reflectance of crystal part) of 5%, an Ra (reflectance ofamorphous part) of 20%, and an Ac (absorptivity of crystal part)/Aa(absorptivity of amorphous part) of 1.3, which are the same as those ofa typical absorptivity adjusting LtoH structure.

[0251] In addition, in the film structure, the phase difference betweenthe reflected light beams of the crystal part and amorphous part wasadjusted to be zero. All of layers other than the recording layer 324were formed by typical sputtering method and conditions for use intypical experiments. That is, the sputtering method was the magnetronsputtering method, and the sputter gas was pure Ar gas. In addition, thegas pressure was 0.67 Pa, and the power inputted to the target was tensto hundreds W. Moreover, the substrate was a non-bias substrate.

[0252] On the other hand, the technique and conditions for depositingthe GeSbTe recording layer 324 was special technique and conditions inthis preferred embodiment. Referring to FIG. 10, a method for formingthe recording layer will be described below. First, a substrate 321, onwhich a semitransparent layer 322 and a lower interference layer 323have been formed, is introduced into a sputter chamber 311 for forming arecording layer, and mounted on a substrate holder facing a target 313shown in FIG. 10. Then, the sputter chamber 311 is evacuated, and amixed gas of Kr and Xe containing 20% Kr was introduced from a gassupply system 316 at a total flow rate of 200 sccm. Then, after theconductance of an exhaust system 317 was adjusted so that the gaspressure in the sputter chamber 311 was adjusted to be 6.7 Pa, an RFpower of 50 W was inputted to a sputter source 314 by a power source315, and simultaneously, a surface migration control system 318 wasoperated to input a weak RF power of 10 W to the substrate tobias-magnetron-sputter the GeSbTe target 313 for five minutes to form aGeSbTe recording layer 324 having a thickness of 10 nm on the lowerinterference layer 323.

[0253] The different points from a typical sputtering were as follows:

[0254] (1) The mixed gas of Kr and Xe was used as a sputter gas to allowGeSbTe sputter particles to easily lose energy in a gaseous phase;

[0255] (2) The gas pressure was set to be high to promote the cooling ofthe GeSbTe sputter particles in the gaseous phase, and the energy of thegas ions being incident on the target was decreased to decrease theenergy when the sputter particles were emitted from the target;

[0256] (3) The power inputted to the sputter source was set to berelatively low to more decrease the energy when the sputter particleswere emitted; and

[0257] (4) The weak bias was applied to the substrate to cause the gasions to be incident on the substrate during the thin-film deposition soas not to thermally damage the polycarbonate substrate 21 to increasethe surface migrate time for the sputter particles on the substrate.

[0258] The above described different points are effective in thedecrease of the energy of the sputter particles being incident on thesubstrate and in the increase of the transition time from a randomliquid phase state to a solid phase state on the substrate, i.e., in thedecrease of the cooling rate for the sputter particles. By such amethod, the cooling rate for the GeSbTe sputter particles in the sputterdeposition process can be decreased from 10¹² K/sec in the conventionalmethod to an order of 10¹⁰ K/sec during an optical recording operation,so that the amorphous state immediately after the thin-film depositioncan approximate to the amorphous state during the optical recordingoperation.

[0259] In order to cause the amorphous state immediately after thethin-film deposition to approximate to the amorphous state during theoptical recording operation, it is not always required to carry out allof (1) the selection of a sputter gas suitable for the above describedmaterial of the recording layer, (2) the increase of the gas pressure soas to be suitable for the above described material of the recordinglayer, (3) the decrease of the sputter power so as to be suitable forthe above described material of the recording layer, and (4) theapplication of the substrate bias so as to be suitable for the abovedescribed material of the recording layer. These methods may be suitablycombined.

[0260] In addition to the above described methods, effective methodsinclude (5) a method for heating the substrate to about tens ° C. duringthe thin-film deposition, and (6) a method for promoting the surfacemigration of the sputter particles on the substrate or for ionizing thesputter particles emitted from the target surface to pass the sputterparticles through a deceleration field, by a method for providing anauxiliary ion source to irradiate the surface of the substrate with anion shower during the thin-film deposition or the like.

[0261] Moreover, effective methods include (7) a method for increasingthe distance between the target and the substrate, and (8) a method fordecreasing the energy of the sputter particles being incident on thesubstrate by a method for eccentrically arranging the substrate withrespect to the target to deposit a film by only sputter particlesemitted obliquely from the target surface.

[0262] By suitably combining these methods, the amorphous stateimmediately after the sputter deposition can approximate to theamorphous state during the optical recording operation. In addition,while the recording layer 24 has been deposited by the most practicalsputtering method in all of the above described examples, it iseffective to apply the vacuum deposition method, the gas depositionmethod, the MBE (molecular beam expitaxy) method, the plasma CVD(chemical vapor deposition) method, the MOCVD (metal-organic chemicalvapor deposition) method or the like to the deposition of the recordinglayer in order to set the energy of the material particles of therecording layer being incident on the substrate to be low.

[0263] On the GeSbTe recording layer 324 produced using (1) a Kr—Xemixed gas, (2) a high pressure atmosphere, (3) a low power and (4) thesubstrate bias magnetron sputter method, an upper interference layer 325and a reflective layer 326 were sequentially formed by a typicalmagnetron sputtering method to be ejected from the sputter chamber. Thisdisc having a five-layer construction is applied on a polycarbonatesubstrate having a diameter of 120 mm and a thickness of 0.6 mm, onwhich no film is provided, via a UV-curing adhesive layer, to beprepared as a sample for verifying the advantages of this preferredembodiment.

[0264] As a comparative example, a disc having the same filmconstruction as that in the above described optical disc was prepared bya different producing method. The comparative disc was formed byinitial-crystallizing a recording layer with an elliptical beam inaccordance with a conventional producing process after the recordinglayer was formed on typical sputtering conditions by a typicalsputtering method to form an applied structure. The conditions forsputtering the recording layer for use in the comparative example werethe same as the conditions for sputtering films other than the recordinglayer in the above described preferred embodiment. The magnetronsputtering method was used, and the sputter gas was pure Ar gas. Inaddition, the gas pressure was 0.67 Pa, and the power inputted to thetarget was tens to hundreds W. Moreover, the substrate was a non-biassubstrate.

[0265] The optical recording of the optical disc in this preferredembodiment and the comparative disc was carried out by the followingmethod. That is, a disc evaluating system having a recording and/orreproducing optical system having a wavelength of 650 nm and an NA ofobjective lens of 0.6 was used. The e⁻² diameter of a laser spot isabout 0.9 μm. On the other hand, the track pitch of the disc is 0.74 μmwhich is smaller than the e⁻² diameter of a laser spot. By using suchlaser beams, with respect to the data part, a 8/16 modulating randompattern signal having a linear velocity of 6 m/sec and a shortest bitlength of 0.41 μm/bit was recorded on adjacent land and groove, each ofwhich has 10 tracks.

[0266] The inventor observed the recording states of the disc of FIG. 12in this preferred embodiment, on which the optical recording operationwas carried out without carrying out the initial crystallization, andcomparative disc, by a transmission electron microscope (TEM). In orderto carry out the TEM observation, the recording part was cut to preparea small piece sample, and the counter substrate together with thecounter substrate was peeled off. Thereafter, the substrate wasdissolved to be removed, and only the medium film part was picked up ona mesh. Then, Au of the first layer was removed by ion milling.

[0267]FIG. 13 is a schematic diagram showing a pattern after initiallyrecording the optical disc in this preferred embodiment.

[0268]FIG. 14 is a schematic diagram showing a pattern when thehundredth overwrite operation (OW100) is carried out on the optical discin this preferred embodiment. In both figures, L_(i-1) and L_(i) denotea number i-1 land track and a number i land track, respectively, and Giis a number i groove track. In addition, A denotes an amorphousrecording mark part, C denoting a crystalline intermark part, CBdenoting a crystal band width, AB denoting the width of an amorphousband between crystal bands, and MW denoting the width of a mark.Although MW is not shown in FIG. 13, MW corresponds to CB in the patternof FIG. 13.

[0269] On the other hand, in the comparative disc produced by theconventional method, it is required to carry out initialcrystallization. Therefore, after the initial recording and after theOW, there was a pattern wherein the amorphous band AB part does notexist in FIG. 14 and amorphous recording marks are scattered in anetwork in a crystalline state.

[0270] In addition, on the disc in this preferred embodiment, randomdata were recorded at a high speed of 12 m/s, and the TEM observation ofthe recording pattern was carried out. As a result, a patternapproximating to that of FIG. 13 was obtained even after the OW.

[0271] These observation results can be interpreted as follows. That is,during a recording operation, a part serving as a recording mark melts,and the temperature of the surrounding part is raised to itscrystallizable temperature zone. If the linear velocity is low or if thenumber of OW operations is large, the integrated time that thetemperature of the mark edge is raised to its crystallizable temperaturezone is long, so that a crystal part is produced around the amorphousmark. Therefore, the disc in this preferred embodiment has a patternapproximating to that shown in FIG. 14 as the linear velocity decreasesand as the number of OW operations increases, and a patternapproximating to that shown in FIG. 13 as the linear velocity increasesand as the number of OW operations decreases.

[0272] That is, in either case, the optical disc in this preferredembodiment is a phase change optical disc wherein the reflectance of acrystal part is adjusted to be lower than the reflectance of anamorphous part viewed from an optical detection side and wherein arecording state has a pattern, in which an amorphous mark row having awidth narrower than or equal to the width of a crystal band (anamorphous band is formed between adjacent crystal bands) than the widthof a track is formed in the crystal band, which is narrower than thewidth of a track, the state of the phase change recording layer of anaddress part being an amorphous state.

[0273] While the examples of this preferred embodiment have beendescribed, this preferred embodiment should not be limited thereto.

[0274] For example, in the above described example, a four-layerstructure of ZnS—SiO₂/GeSbTe/ZnS—SiO₂/Al or AlMo film deposited on asubstrate by sputtering, or a five-layer structure, in which an Ausemitransparent film is provided in the four-layer structure, isillustrated as a medium having a film structure of Rc<Ra. However, anyone of medium of Rc<Ra may be used in this preferred embodiment. Inaddition, the material and thickness of each of the layers, and themethod and conditions for depositing films other than the recording filmshould not be limited, except for the conditions for sputtering andinitializing the recording layer which are important in this preferredembodiment.

[0275] For example, in the case of a five-layer structure, thesemitransparent layer may be formed of silver (Ag), copper (Cu), silicon(Si) or a film having a structure wherein fine metal particles aredispersed in a dielectric matrix, in place of Au. In addition, thematerial of the interference layer may be suitably selected fromdielectric film materials, such as Ta₂O₅, Si₃N₄, SiO₂, Al₂O₃ and AlN, inplace of ZnS—SiO₂, and the material of the recording layer may besuitably selected from chalcogen film materials, such as InSbTe,AgInSbTe and GeTeSe, in place of GeSbTe. The material of the reflectivelayer may be suitably selected from Al alloy film materials, such asAlCr and AlTi, in place of AlMo.

[0276] Moreover, while the optical disc has been described as an exampleof an optical recording medium in the above described example, thispreferred embodiment should not be limited thereto. For example, thispreferred embodiment may be applied to any one of various opticalrecording media, such as an optical recording card, to obtain the sameadvantages as those of this preferred embodiment.

[0277] This preferred embodiment is applied in the above described formto have the undermentioned advantages.

[0278] First, according to this preferred embodiment, it is possible toimprove the quality of address signals and servo signals of a phasechange recording medium wherein the reflectance of a crystal part is setto be lower than the reflectance of an amorphous part, and it ispossible to reduce the jitter of a data part from the first recordingoperation.

[0279] In addition, according to this preferred embodiment, it ispossible to provide an optical recording medium, which can be usedimmediately in an as-deposited state without passing through anadditional process for initial-crystallizing a recording film and whichhas excellent recording characteristics and high repeatable overwritecharacteristics.

[0280] As a result, it is possible to reduce equipment investments,operation costs and time necessary for initial crystallization, and itis possible to provide an inexpensive, high performance opticalrecording medium.

[0281] As described above, according to this preferred embodiment, it ispossible to provide a phase change optical recording medium which has ahigher performance than those of conventional media and which cansimplify the producing process thereof. This has great industrialmerits.

[0282] (Fourth Preferred Embodiment)

[0283] The fourth preferred embodiment of the present invention will bedescribed below. This preferred embodiment is characterized in that therecording layer of a phase change recording medium has a unique range ofthermal conductivity.

[0284] In this preferred embodiment, there is provided a phase changerecording medium which has a recording layer, wherein a phase changebetween a crystal phase and an amorphous phase is caused by irradiationwith light from the outside, as means for accomplishing the abovedescribed object, and which is characterized in that the thermalconductivity of the amorphous state is 0.8 W/mK or more and 6 W/mK orless and that the recording layer has an address part and a data part,the state of the address part being substantially an amorphous state.

[0285] Data are not written on the address part of the phase changerecording medium by the user. That is, the initial state of the addresspart is held. In this preferred embodiment, a recording layer depositedby a method, which will be described in detail later, has the abovedescribed thermal conductivity, and can record data immediately in anas-deposited state, so that it is not required to carry out an initialcrystallizing process. That is, the recording medium in this preferredembodiment is characterized in that the address part has the abovedescribed construction even if it is used by the user after it isproduced.

[0286] Furthermore, throughout the specification, “substantiallyamorphous state” means an optically amorphous state. For example, thelight reflectance, which is important as the characteristic of arecording medium, is closer to the reflectance of an amorphous mark thanthe reflectance of a crystal space.

[0287] In addition, in this preferred embodiment, there is provided aphase change recording medium having a recording layer wherein a phasechange between a crystal phase and an amorphous phase is caused byirradiation with light from the outside, the recording layer having anaddress part and a data part, the state of which is substantially anamorphous state in an unrecorded state before recording data, thethermal conductivity of the amorphous state being 0.8 W/mK or more and 6W/mK or less.

[0288] That is, since it is not required to carry out any initialcrystallizing steps in order to produce a medium in this preferredembodiment, the data part of the medium in this preferred embodiment hasthe above described structural characteristic in an unused state beforedata are written on the data part.

[0289] The thermal conductivity is more preferably 2 W/mK or more and 4W/mK or less.

[0290] In addition, the principal component of the recording layer ispreferably GeSbTe or AgInSbTe.

[0291] The inventor found that a recording medium, wherein the thermalconductivity of an as-deposited amorphous part is adjusted to be in theabove described range, has good recording characteristics even if dataare recorded in an as-deposited amorphous state without the need of anyinitial crystallizing steps.

[0292] The phase change recording layer in the above described preferredembodiment is characterized in that the thermal conductivity of anamorphous part in an initial state, i.e., a state as depositing therecording layer by a sputtering method or the like, is lower than thethermal conductivity of an amorphous part of a conventional phase changerecording medium and that the control range is wide. Since the thermalconductivity is set to be lower than that of the conventional phasechange recording medium, the temperature of the amorphous part tends torise even if Aa is adjusted to be optically small, so that it ispossible to obtain good erase characteristics. In addition, since thephase change recording layer in this preferred embodiment can controlthe thermal conductivity in a wider range in accordance with theproducing method, it is possible to improve the degree of freedom inthermal response design.

[0293] Basically, this preferred embodiment relates to materials.However, since a method for producing a phase change recording medium isimportant in order to obtain a predetermined thermal conductivity, amethod for producing a phase change recording medium in this preferredembodiment will be briefly described.

[0294] A recording layer for use in a phase change recording operationis deposited usually by a sputtering method. Immediately after thethin-film deposition, the state of the recording layer is an amorphousstate. The sputtering method is a technique for producing apredetermined film by allowing sputter particles (gaseous phase), whichare sputter-emitted from a target surface by a high-energy argon (Ar)ion bombardment, to arrive at the surface of a substrate at random tomigrate the surface in a random state (liquid phase), and thereafter, byallowing the state of the phase to be a solid phase serving as a film.The cooling rate during the transition from the gaseous phase to thesolid phase is usually about 10¹² K/sec. That is, it is guessed that thetime required for a random state of several eV (tens of thousands K) tobe changed to a solid phase at room temperature is about 10 nanoseconds,and that the time required to pass through a temperature zone between amelting point and a crystallizing temperature is about 1 nanosecond atthe most.

[0295] On the other hand, the time to crystallize a GeSbTe or InSbTerecording layer is tens nanoseconds. The condition for allowing a filmto be crystallized is that the time to crystallize the film is shorterthan the crystallization holding time, so that the state of therecording layer immediately after the sputter deposition is an amorphousstate. It is herein important that the amorphous state immediately afterthe thin-film deposition is different from the amorphous state formed bythe optical recording operation. Because the cooling rate during theoptical recording operation is typically about 10¹⁰ K/sec, which issmaller than the cooling rate in a sputter deposition process by abouttwo digits, although it depends on the linear velocity of a beam and thelayer structure of a medium. The difference between the cooling ratesfrom a molten state (which means both of the migration process insputter deposition and the melting process during the optical recordingoperation) is reflected in randomness. That is, as the cooling rate ishigher, randomness is higher, and when the cooling rate is low, it has afine structure having a short-range order although it is microscopicallyrandom.

[0296] This difference in fine structure is reflected in thermalconductivity. In general, the thermal conductivity in a random state islower. The reason for this is that scattering is great in a randomsystem in either case where thermal conduction is caused by latticevibration or electronic conduction. Therefore, comparing the amorphousstate with the crystalline state, the thermal conductivity in theamorphous state is clearly lower than that in the crystalline state.When the short-range order (which may be called nano-crystal) iscontained in the amorphous state, it is considered that the thermalconductivity has an intermediate value between the amorphous state andthe crystalline state if a simple weighted means is derived.

[0297] However, after the detailed measurement of thermal conductivitywas carried out, the inventor found that the thermal conductivity of anamorphous material containing a nano-crystal was lower than that in anamorphous material containing no nano-crystal. Although some reasons forthis are guessed, a strong reason is the amorphous part of the amorphousmaterial containing the nano-crystal contains a larger number ofdangling bonds in the vicinity of the nano-crystal. Another reason isthat there is a mechanism wherein head is absorbed by a nano-crystalpart having a lower heat capacity than that of an amorphous part toinhibit the whole thermal conduction.

[0298] A concrete method for forming a phase change recording mediumhaving a recording layer having a thermal conductivity in this preferredembodiment is to lower the cooling rate of sputter particles in asputter process to cause an amorphous state immediately after thethin-film deposition to approximate to an amorphous state formed by anoptical recording operation. In order to cause the amorphous stateimmediately after sputtering to approximate to the amorphous stateformed by the optical recording operation, the energy of sputterparticles being incident on a substrate is decreased, or the time forsurface migration is increased. More specifically, there is effectivelya method for using, as a sputter gas, krypton (Kr) gas, xenon (Xe) gasor the mixed gas thereof, which have a greater cooling effect on GeSbTesputter particles than generally used argon (Ar) gas, a method forlightly applying a bias to a substrate to promote surface migration, orthe like. Immediately after the sputter deposition, the state of a discthus formed approximates to an amorphous state formed by an opticalrecording operation. Therefore, the disc has a low thermal conductivitypeculiar to the nano-crystal containing amorphous material.

[0299] Referring to the accompanying drawings, the examples of thispreferred embodiment will be described below.

[0300]FIGS. 15 and 16 are conceptual views showing examples of thesectional structure of a phase change recording medium in this preferredembodiment. In these figures, the same reference numbers are used formembers having the same functions. FIG. 15 shows a medium having atypical LtoH structure, and FIG. 16 shows a medium having a typical HtoLstructure (Rc>Ra).

[0301] In FIGS. 15 and 16, reference number 401 denotes a polycarbonatesubstrate having tracking grooves, 402 denoting a semitransparent film,403 denoting a lower interference layer, 404 denoting a recording layer,405 denoting an upper interference film, and 406 denoting a reflectivefilm.

[0302]FIG. 17 is a conceptual plan view showing an example of arecording medium. That is, this figure shows the plan structure of aDVD-RAM, the lower side of this figure showing the whole structure ofthe DVD-RAM, and the upper side thereof showing an enlarged view of apart of the DVD-RAM. On the disc, “land tracks” and “groove tracks” arealternately formed on the disc. The disc is radially divided into aplurality of “sectors”. On the top part of each of the sectors, a“header part”, i.e., an “address part”, is provided. On the “headerpart”, sector information, such as address, is provided as a pre-pitstring. The data recording and/or reproducing operation is carried outon the “header part”, and it is carried out on the “data part”.

[0303] Therefore, the crystalline state or amorphous state of the“header part” of the recording medium does not change after the productis shipped. In other words, when the initial state of the medium is anamorphous state, the state of the “header part” remains being theamorphous state. On the other hand, when the medium passes through acrystallization step similar to conventional media, the state of the“header part” remains being the crystalline state.

[0304] As will be described later, the recording medium in thispreferred embodiment does not pass through the initial crystallizingstep, so that the state of the “header part” is substantially anamorphous state, and the thermal conductivity thereof is in a uniquerange which is different from the conventional range.

[0305] Alternatively, when the state of the recording medium in thispreferred embodiment is an unused state, the state of the “data part” isalso substantially an amorphous state, and the thermal conductivitythereof is in a unique range which is different from the conventionalrange.

[0306] The phase change recording medium in this preferred embodimentmay be produced by the following procedure. First, a polycarbonatesubstrate 1 may be produced by a typical mastering process for anoptical disc substrate. For example, the polycarbonate substrate 1 has athickness of 0.6 mm, a diameter of 120 mm, a track pitch of 0.6 μm and agroove depth of 70 nm.

[0307] Each of layers may be formed using, e.g., a magnetron sputteringsystem. In the case of FIG. 15, for example, an Au semitransparent film402 having a thickness of 10 nm, a lower ZnS—SiO₂ interference film 403having a thickness of 80 nm, a GeSbTe recording layer 404 having athickness of 20 nm, an upper ZnS—SiO₂ interference film 405 having athickness of 30 nm, and an Al alloy reflective film 406 having athickness of 100 nm are sequentially deposited by sputtering on thegroove surface of a substrate 401 in that order. In the case of therecording medium of FIG. 16, for example, a lower ZnS—SiO₂ interferencefilm 403 having a thickness of 120 m, a GeSbTe film 404 having athickness of 15 nm, an upper ZnS—SiO₂ interference film 405 having athickness of 20 nm and an Au reflective film 406 having a thickness of10 nm are deposited by sputtering.

[0308] After the respective layers are deposited, the medium is ejectedfrom the sputtering system. Thereafter, the medium is mounted on, e.g.,a blank substrate, so that it is possible to obtain a phase changerecording medium.

[0309] In this preferred embodiment, the conditions for forming therecording layer and the upper and lower interference films are adjustedin the above described producing procedure to control the lower thermalconductivity of the amorphous material than those of conventional media,and the thermal conductivity of the amorphous material in a wide range.Specifically, any one of the following conditions or suitablecombinations thereof is adopted.

[0310] (1) When a recording layer is formed, any one of Kr, Xe and Kr—Xegases, which have a high cooling effect on Ge, Sb, Te sputteringparticles, is used in place of Ar gas which is used for conventionalmethods.

[0311] (2) In order to promote the cooling effect in a process fortransporting sputter particles, the gas pressure is set to be higherthan 0.25 to 0.67 Pa, which is a typical value in conventional methods.

[0312] (3) The input to a GeSbTe target is set to be low to decrease thecathode fall voltage to suppress the emission energy of sputterparticles.

[0313] (4) During the thin-film deposition, a weak bias is applied to asubstrate holder to increase the migration time of sputter particles ona substrate.

[0314] (5) During the thin-film deposition, a substrate is heated in atemperature zone less than the thermal modification temperature of apolycarbonate substrate.

[0315] By adopting the above described conditions, the cooling rate forsputter particles on the substrate can be decreased, so that fine nucleihaving a short-range order can be produced in an as-deposited recordinglayer. Thus, the thermal conductivity of the as-deposited amorphous partcan be lower than the conventional thermal conductivity. Simultaneously,by controlling the cooling rate of sputter particles on the substrate,the thermal conductivity of the amorphous part can be controlled in awide range.

[0316] The formation of the short-range order in the recording layer canbe carried out by improving the conditions for depositing the upper andlower interference films in addition to the above described adjustmentof the conditions for depositing the recording layer. Specifically, theinterference film is formed at a lower gas pressure than a typical gaspressure, and the sputter input power is set to be higher than a typicalpower of hundreds W to thousands W. The interference film thus formedhas a compressive stress to promote the volumetric shrinkage of therecording layer. In the recording layer, the volume of the crystal partis smaller than that of the amorphous part having the same number ofatoms as that of the crystal part. Therefore, if the volumetricshrinkage is promoted by the upper and lower films, a short-range orderhaving a smaller volume than that of an-amorphous part is easily formedin the recording layer.

[0317] The inventor formed phase change recording media shown in FIGS.15 and 16 by the above described method. In addition, in order toclarify their effects, a comparative phase change recording mediumhaving the same sectional structure was produced by the conventionaldeposition method as a comparative example. The conventional depositionmethod means a method for forming a recording layer using Ar gas at agas pressure of about 0.4 Pa and a sputter input of hundreds W and forforming upper and lower interference films at a gas pressure of about0.67 Pa and a sputter input of hundreds W.

[0318] The medium having the structure of FIG. 15 produced by thedeposition method in this preferred embodiment will be hereinafterbriefly referred to as a “disc 1” in this preferred embodiment, and themedium having the structure of FIG. 16 produced by the deposition methodin this preferred embodiment will be hereinafter briefly referred to asa “disc 2”. In addition, the medium having the structure of FIG. 15produced by the conventional deposition method will be hereinafterbriefly referred to as a “comparative disc 1”, and the medium having thestructure of FIG. 16 produced by the conventional deposition method willbe hereinafter briefly referred to as a “comparative disc 2”.

[0319] The measurement of the thermal conductivity of the recordinglayer of the conventional phase change recording medium as compared withthe phase change recording medium produced by the above describedtechnique in this preferred embodiment was carried out by the“high-speed time-resolved (picosecond) thermoreflectance technique”.This is a technique developed by Baba et al., Ministry of InternationalTrade and Industry, Industrial Technology Department, National ResearchLaboratory of Metrology. Measurement System Department, MeasurementInformation Section. The details of the technique is disclosed in, e.g.,Proc. Thermophysical Properties 17, p43, Prc. EUROTHERM '57 “MicroscaleHeat Transfer”.

[0320] This measuring method utilizes light and heat material propertiesthat the reflectance of a material depends on the temperature of thematerial. The variation in reflectance of a material with respect totemperature depends on the kind of the material. For example, in thecase of Al, the variation in absolute reflectance is about 10⁻⁵ (K−1).This phenomenon can be physically explained as the variation in thermalreflectance caused by the fact that the thermal oscillation of a latticehas a slight influence on the electronic state thereof. In thismeasuring method, a pump light beam having a pulse width of aboutpicoseconds is used as means for heating the surface of a material. Whenthe material is irradiated with a pulse light beam, the surfacetemperature of the material rises, and the reflectance thereof varies,generally increases. When the pump light beam is turned off, heat isdiffused in a direction of the thickness of the sample from the surfacethereof, so that the surface temperature falls and the reflectancedecreases. In the case of a thin-film sample having a thickness of tensnm, the time constant of a thermal diffusion in a direction of thethickness of the film depends on the thermal conductivity of the film,and has a value between tens picoseconds and several nanoseconds. Whenthe variation in surface reflectance after turning off the pump lightbeam is monitored by a probe light beam having a low power, by which thesurface is not substantially heated, the time constant of a thermaldiffusion in a direction of the thickness of the film is measured. Ifthis time constant is converted into the thermal permeability in adirection of the thickness of the film and if the resulting thermalpermeability is converted into a thermal conductivity (which is assumedas a linear thermal conduction in a direction of the thickness of thefilm), it is possible to derive the value of the heat conductivity.

[0321] The high-speed time-resolved (picosecond) thermoreflectancetechnique method is the only technique capable of accurately measuringthe thermal conductivity of a thin-film sample having a thickness oftens nm. The reliability of the measured value is far higher than, e.g.,the “Alternating Current (AC) Calorimetric method” which has been usedwell. Originally, the Alternating Current (AC) calorimetric method wasdeveloped in order to measure the thermal conduction of a film samplehaving a thickness of hundreds μm. However, there was no measuringmethod suitable for a thin-film sample having a thickness of tens tohundreds nm, so that the Alternating Current (AC) calorimetric methodhad to be applied to a thin-film. An example of the thermal conductanceof a film material for use in a phase change recording medium, which wasmeasured by the Alternating Current (AC) calorimetric method, isdisclosed in, e.g., Jpn. Appl. Phys. 28-3, pp.123-128. As the measuredvalue of the thermal conductivity of an amorphous part of a GeSbTerecording layer relating to this preferred embodiment, a value of 0.58W/mK is clearly described. This measured value is very doubtful inrespects of the facts that this measured value is lower than the thermalconductivity of ZnS—SiO2 disclosed in the same literature, that themeasured value is the same as the thermal conductivity of the crystalpart of the GeSbTe film, and that the measured value is far lower thanthe bulk thermal conductivity of each of Ge, Sb and Te.

[0322] The inventor has reexamined measurement by the AlternatingCurrent (AC) calorimetric method. However, the measured value of asample having a thickness of tens nm for use in an actual phase changerecording medium varied widely, so that it was not possible to carry outsignificant measurement at all. Moreover, also with respect to themeasurement of a sample having a thickness of tens μm, to which theAlternating Current (AC) calorimetric method was able to be applied, thedispersion in measured value exceeded plus and minus tens %, so that itwas difficult to acquire reliable data.

[0323] On the other hand, as described above, the high-speedtime-resolved (picosecond) thermoreflectance technique method isdesigned to carry out the high-speed time-resolved measurement of thethermal diffusion in a direction of the thickness of a sample, and toobserve the variation in reflectance of the surface of the sample by aprobe light beam in picosecond order after heating the surface of thesample by a pump light beam. This is a technique for precisely measuringbehavior wherein when heat is diffused in a direction of the thicknessof a sample after heating the surface of the sample, the surfacetemperature thereof falls and the thermal reflectance thereof decreases.This is a technique useful for all of film materials although theprecision of measurement is particularly high with respect to a materialhaving a high thermal reflectance, e.g., Al. Because if an Al thin-filmis coated on the surface of, e.g., even a transparent thin-film materialhaving a low reflectance, the thermal conductivity of the transparentthin-film material can be known by examining the time varied temperatureof the surface of the Al coated film in accordance with the thermaldiffusion in a direction of the thickness of the transparent thin-filmmaterial.

[0324] The inventor carried out the detailed measurement of the thermalconductivity of a thin-film sample having a thickness of tens nm. In themeasurement of the thermal conductivity, the disc samples of FIGS. 15and 16 and a sample, wherein only a recording layer was produced on,e.g., an Si wafer, and wherein an Al film used for measuring a thermalreflectance was coated thereon, were used. In the latter, a GeSbTe filmhaving a thickness of 100 nm was formed on an Si wafer by the abovedescribed deposition method in this preferred embodiment or by theconventional deposition method, and an Al film having a thickness of,e.g., 20 nm, was sequentially coated thereon. When the former discsample was used directly, a recording layer was exposed by a method forsticking a tape an Al alloy reflective film to peel off the tape afterpeeling off a stuck counter blank substrate, and an Al film having athickness of about 10 to 20 nm was coated thereon.

[0325] As a result of measurement, the thermal conductivity of theformer was substantially the same as that of the latter in a range of±5%. Throughout the specification, the value of the latter, i.e., thevalue of the sample produced on the Si wafer, is used as the measuredvalue.

[0326] The thermal conductivity of the amorphous state of the recordinglayer produced by the conventional method was typically 7 (W/mK), and inthe range of from 6.8 to 7.2 (W/mK) in accordance with the formationconditions. In addition, the thermal conductivity of the crystallinestate was in the range of from 8.8 to 10.1 (W/mK) in either case of theconventional recording layer or the recording layer in this preferredembodiment, and was about 1.3 to 1.4 times as large as the thermalconductivity of the amorphous state of the conventional recording layer.

[0327] On the other hand, the thermal conductivity of the recordinglayer in this preferred embodiment was in the range of from 0.8 to 6(W/mK) in the as-deposited state, i.e., in the amorphous state. It isconsidered that the fluctuation in value in this rage depends on themethod for producing the recording layer, i.e., the size and contentratio of fine nuclei contained in the amorphous material.

[0328] Since it was guessed that the recording layer in this preferredembodiment has fine nuclei in the amorphous state, it was predicted thatthe thermal conductivity thereof was an intermediate value between thethermal conductivity of the amorphous state of the conventionalrecording layer and the thermal conductivity of the crystalline state.On the other hand, it is guessed that the reason why the recording layerin this preferred embodiment has a far lower value in the amorphousstate than those of the crystalline state and amorphous state of theconventional recording layer is that the thermal resistance is high dueto the dangling on the interface between fine nuclei and so forth asdescribed above.

[0329]FIG. 18 is a graph showing the relationship between the measuredvalue of thermal conductivity (κ) and DC erasing rates measured using adisc sample having the structure of FIG. 15.

[0330] The measurement of the DC erasing rates was carried out by thefollowing method. That is, the disc having the structure of FIG. 15 wasset in an evaluating system having a light source having a wavelength of650 nm and an objective lens having an NA of 0.6. Then, the disc wasrotated at a linear velocity of 10 m/sec, and a mark corresponding to11T was recorded at the optimum recording power (10 to 13 mW in thiscase). Thereafter, the CNR of the disc was measured, and the disc wasirradiated with an erase power (4 to 6 mW) as DC to measure thedecreased amount of the 11T carrier level. The initial crystallizationof the recording layer was not herein carried out with respect to bothof the disc in this preferred embodiment and the conventional disc.

[0331] In FIG. 18, black circles denote the thermal conductivity κ andDC erasing rate of the phase change recording medium in this preferredembodiment, and a write circle denotes those of the conventional phasechange recording medium. In the conventional disc, the DC erasing ratewas about 5 dB at the most, so that the erasing characteristic thereofwas very bad. On the other hand, in the disc in this preferredembodiment, it was possible to obtain a high erasing rate of 35 dB orhigher when κ was in the range of from 0.8 to 6 W/mK. In particular,when κ was in the range of from 2 to 4 W/mK, it was possible to obtain avery high erasing rate of 40 dB or higher. It is considered that thereason for this is that the time for the temperature of the phase changerecording medium in this preferred embodiment to be held to a highertemperature is long even if the optical absorptivity is the same sincethe thermal conductivity κ of the amorphous state of the recording layerof the phase change recording medium in this preferred embodiment isadjusted to be lower than that of the conventional medium.

[0332]FIG. 19 is a graphs showing the relationship between 3T-CNR andthermal conductivity κ in the first recording operation withoutinitialization. Also in this figure, black circles denote data in thispreferred embodiment, and a white circle denotes data in theconventional medium. As can be seen clearly from FIG. 19, the CNR of theconventional medium is low, about 20 dB, whereas the CNR of the phasechange recording medium in this preferred embodiment is very high, about52 dB, from the first recording operation in a non-initialized state. Itis guessed that the reason for this is that the recording layer in thispreferred embodiment has fine nuclei of a short-range order in theamorphous material even in the as-deposited state, and the formation ofa crystal space proceeds at a high speed even in the first recordingoperation in the as-deposited state. Then, in the medium in thispreferred embodiment, it is considered that the presence of fine nucleiis reflected in the thermal conductivity in accordance with the abovedescribed mechanism.

[0333] As can be seen from the foregoing, the phase change recordingmedium in this preferred embodiment does not need the initializationstep after the thin-film deposition of the recording layer, and theinitial state in the as-deposited state can be used immediately as anamorphous material.

[0334] Also with respect to the HtoL medium shown in FIG. 16, the sameevaluation as the above described evaluation was carried out, so thatthe same results were obtained in the range of a difference of about 2dB. That is, it was found that this preferred embodiment was effectivein the medium having either of the LtoH or HtoL structure. However, whenthe HtoL medium is used in a non-initialized state, the initialreflectance thereof is low, so that the qualities of address signals andservo signals are lower than those when the LtoH medium is used withoutinitialization. Therefore, when the recording medium of FIG. 16 is used,it is desired to the reflectance of the amorphous part to be high, about10%, to adjust so as to stabilize servo.

[0335] In the above described example, the GeSbTe film has be used asthe recording layer. However, this preferred embodiment may be appliedto any films containing other elements added to GeSbTe. In that case,the value of the thermal conductivity κ was shifted by only severalpercents, and the approximately effective range of κ was 0.8 to 6 W/mK.

[0336] While the GeSbTe recording layer has been used in the abovedescribed example, this preferred embodiment may be similarly applied toan AgInSbTe recording layer. That is, a typical composition of GeSbTe isapproximately Ge:0.22, Sb:0.22, Te:0.56, whereas a typical compositionof AgInsbTe is approximately, e.g., Ag:0.08, In:0.13, Sb:0.49, Te:0.30,as disclosed in Jpn. Appl. Phys., 32, pp.5241-5247. That is, bothcontained Sb and Te as principal components, and the same goodcharacteristics as those of GeSbTe in a range of thermal conductivity κshifted by about 10%.

[0337] More specifically, the material of the recording layer may besuitably selected from chalcogen metal compounds, such as Ge—Sb—Te andAg—In—Sb—Te, and materials suitably containing a very small amount ofCr, V, N or the like, in place of the above described materials. Thedesired composition range of the phase change recording medium is acomposition range which copes with both the rapid crystallization at atemperature higher than a crystallizing temperature and the thermalstability of the amorphous state at room temperature.

[0338]FIG. 20 is a graph showing an example of the distribution incrystallizing time in a GeSbTe ternary alloy system. In this figure, itis possible to carry out crystallization at a higher speed as thecrystallizing time is shorter.

[0339]FIG. 21 is a graph showing an example of the distribution incrystallizing temperature in a GeSbTe ternary alloy system. In thisfigure, the amorphous state is more thermally stable as thecrystallizing temperature is higher.

[0340] Data of FIGS. 20 and 21 are disclosed in, e.g., J. Appl. Phys.,69(5), pp.2849-2856 (1991). As can be seen from these figures, thecomposition range capable of coping with both the rapid crystallizationand the stability of the amorphous state is a composition range whereineach of the composition ranges of Ge, Sb and Te is ±5 at % about acomposition segment, on which a ratio of GeTe:Sb₂Te₃ is in the range offrom 5:2 to 1:6.

[0341]FIG. 22 is a graph showing a desired composition range in anAg—In—Sb—Te four-element alloy. That is, in this figure, x and y of acomposition formula expressed by (AgSbTe₂)×(In_(1−y)Sby)_(1−x) areplotted on the axes of abscissas and ordinates of the graph. In thisfigure, the composition range shown by B is desired for a recordinglayer, and the composition range shown by A is more desired as thematerial of a recording layer. That is, the composition range expressedby x=0.37-0.42 and y=0.62-0.79.

[0342] As described in detail above, according to this preferredembodiment, it is possible to carry out a recording operation at a highCNR immediately from an as-deposited state, so that it is possible toremove an initial crystallizing step from a process for producing aphase change recording medium. As a result, it is possible to reduce theproducing costs and to cause phase change recording media to spreadwidely.

[0343] (Fifth Preferred Embodiment)

[0344] The fifth preferred embodiment of the present invention will bedescribed below. In this preferred embodiment, there is provided aproducing method which omit an initial crystallizing step by providingunique conditions when the recording layer of a phase change recordingmedium is sputtered.

[0345] That is, there is provided a method for producing a phase changerecording medium having a substrate and a recording film depositedthereon, wherein the relationship between a voltage Vdc applied to atarget and a sputter threshold voltage Vth of a target constituentelement is set to be Vth<Vdc≦10 Vth when the recording film is depositedon the substrate by sputtering.

[0346] In a preferred example of this preferred embodiment, therelationship between the voltage Vdc and the sputter threshold voltageVth is set to be 3 Vth≦Vdc≦8 Vth.

[0347] In addition, an ion density Ni in a negative glow plasma producedduring sputtering is greater than 10¹¹ (cm⁻³).

[0348] In addition, the recording film contains GeSbTe or AgInSbTe as aprinciple component.

[0349] On the other hand, a system for producing a phase changerecording medium in this preferred embodiment is a system for producinga phase change recording medium having a substrate and a recording layerdeposited thereon, the system comprising a target for depositing therecording layer on the substrate by sputtering, a power supply forapplying power to the target to produce a negative glow plasma, and aplasma density increasing means provided for enhancing the density ofthe negative glow plasma.

[0350] In order to accomplish the object of this preferred embodiment,after the inventor studied conditions and methods for sputtering arecording layer so that an as-deposited amorphous state approximates toan amorphous state formed by an optical recording operation after aninitialing process, the above described present invention was made.Typical constituent elements of a target used for forming a phase changerecording layer include Ge, Sb, Te, Ag and In. The sputter thresholdvoltage (Vth) for these elements is approximately 20 eV although itdepends on the kind of element and the kind of sputter gas.Conventionally, in order to increase the thin-film deposition rate, atypical value of the Vdc has been in the range of from 300 V to 600 V,which was 15 to 30 times as large as the value of the Vth. After theinventor carried out experiments in view of the recordingcharacteristics of a recording layer without any initializing processes,particularly in view of the variation in reflectance when the firstrecording operation is carried out without carrying out initialization,while the conditions for sputtering the recording layer are varied indetail, the inventor found it was possible to obtain goodnon-initialized recording characteristics in a far lower range of Vdcthan the conventional Vdc.

[0351] That is, the range of Vdc capable of obtaining practicallysufficient first recording characteristics in a non-initialized state,i.e., an as-deposited state, is Vth<Vdc≦10 Vth.

[0352] In general, the Vdc is a voltage applied between a dischargecathode and a negative glow plasma in a gaseous discharge. Insputtering, the target corresponds to a cathode, and positive ions areaccelerated toward the target in a cathode fall during negative glow tobe incident on the target by energy substantially corresponding to theVdc to cause the material of the target to be sputter emitted. The Vdcexists in both the DC discharge and the RF discharge. In the case of theRF discharge, the Vdc is often called “self-bias voltage”. The Vth is athreshold energy sputter-emitted from the target material, and meansthat the sputter emission does not substantially occur in a regionwherein the energy of ions being incident on the target is less than theVth.

[0353] The inventor examined the details of the variation in finestructure of a recording layer when light irradiation is repeated fromthe as-deposited state before this preferred embodiment was made. Inthis examination, the recording layer was deposited in accordance withthe prior art on the condition of Vdc>10 Vth, specifically on thecondition of Vdc=400 V (Vth will be described later). If theas-deposited amorphous state was repeatedly irradiated with light beamshaving an intensity of crystallizing level, the recording layer isgradually crystallized, and the reflectance is changed from an amorphouslevel to a crystal level. If irradiation is repeatedly carried outhundred times or more, the reflectance is completely changed to thecrystallizing level. The recording layer of a medium having anintermediate reflectance between an amorphous material and a crystal,the recording layer of an as-deposited amorphous material, and therecording layer completely crystallized by repeating light irradiationtwo hundreds times were observed by a high resolution transmissionelectron microscope (TEM).

[0354] As a result, in the recording layer of the as-deposited amorphousmaterial, no fine structure appeared, and the electron beam diffractionpattern was a halo pattern peculiar to the amorphous material. On theother hand, the completely crystallized recording layer was an aggregateof crystal grains having a grain size of about 50 nm, and the electronbeam diffraction was a spotty pattern. These structures are well known.

[0355] On the other hand, it was observed that an intermediate state, inwhich the reflectance of the medium had an intermediate level betweenthe amorphous level and the crystal level, had a structure wherein finenuclei having a size of several nm were scattered in the amorphousmaterial, and it was also observed that the density of nuclei wasincreased and the nuclei grew in accordance with the increase of thenumber of light irradiation operations. From these results, the inventorhas had an idea that a recording operation can be carried out from anas-deposited state if it is possible to form a recording layer having astructure wherein fine nuclei are scattered in an as-deposited state.

[0356] In order to form fine nuclei in an as-deposited amorphous state,conditions and methods for sputtering a recording layer were studied. Asa result, it was found that a recording operation can be carried outfrom an as-deposited state if the Vdc during sputtering is controlled ina range defined in this preferred embodiment. Typical constituentelements of a target used for forming a phase change recording layerinclude Ge, Sb, Te, Ag and In. The sputter threshold voltage (Vth) forthese elements is about 12 to 30 eV although it depends on the kind ofelement and the kind of sputter gas. The values of Vth of the abovedescribed elements with respect to various rare gases are shown in table2. TABLE 2 Sputter Threshold Energy (Vth: eV) Sputter Gas He Ne Ar Kr XeGe 30 28 25 22 19 Sb 25 23 20 18 16 Te 24 23 20 16 15 Ag 20 17 15 14 12In 22 20 18 17 15

[0357] Table 2 show the reported values of sputtering yields, andinterpolated values (Vdc at which a thin-film deposition rate issubstantially “0”) of the results of experiments for examining therelationship between thin-film deposition rates and values of Vdcexamined by the inventor.

[0358] In a case where a multicomponent material and/or a multicomponentsputter gas is used, the arithmetical mean of values of table 1 may beused. In addition, the addition of a very small amount of reactive gas,such as additional element, oxygen, nitrogen or hydrogen, does not havea great influence on the values of table 1.

[0359] Conventionally, a typical value of Vdc has been in the range offrom 400 to 600 V, which have been ten times or more as large as Vth, inorder to increase the thin-film deposition rate. On the other hand, theinventor experimentally manufactured media while varying the value ofVdc during sputtering of a recording layer, and repeated experiments inview of the reflectance after being irradiated with a light beam once.As a result, the inventor has found that it is possible to produce finenuclei in the recording layer to obtain good as-deposited recordingcharacteristics in a far lower range of Vdc than the conventional Vdc.The range of Vdc capable of obtaining significant rapid initializationcharacteristics was Vdc≦10 Vth. Since no film is formed when Vdc is nothigher than Vth, the lower limit of Vdc is Vth.

[0360] The reason why fine nuclei are produced in an as-depositedamorphous material by adjusting the range of Vth to Vth<Vd≦10 Vth is asfollows. That is, as described above, the crystallization of the phasechange recording layer proceeds in a temperature zone which is thecrystallizing temperature thereof or higher and which is less than themelting point thereof. The time for the temperature of the recordinglayer to be held in the temperature zone which is the crystallizingtemperature thereof or higher and which is less than the melting pointthereof will be hereinafter referred to as a “crystallization holdingtime”. If this crystallization holding time is sufficiently longer thanthe “crystallizing time” peculiar to each material of the recordinglayer, the recording layer is completely crystallized, and if thecrystallization holding time is shorter than the “crystallizing time”,the recording layer is hardly crystallized.

[0361] The crystallizing time corresponds to a time constant ofcrystallization in the Arrhenius equation or (Johnson-Mehl-Abrami)equation. In view of a sputtering process, sputter particles (Ge, Sb,Te, Ag, In, dimer thereof, trimer thereof, or the like) emitted from atarget are incident on a substrate in a gaseous phase state havingenergy of several eV (tens of thousands K), to be transformed to a solidphase state serving as a thin-film. Also when the sputter particles arechanged from a gaseous phase state to a solid phase state, acrystallizable temperature zone between less than the melting point andthe crystallizing temperature is passed. However, on the conventionalconditions of Vdc, the energy of sputter particles was very high whenbeing incident on the substrate. Therefore, the cooling rate of sputterparticles on the substrate was very high, and the crystallizationholding time was far shorter than the crystallizing time. Thus, nonucleus existed in the as-deposited state of the phase change recordinglayer formed by the conventional method, no nucleus existed, and theas-deposited state was an amorphous state having vary high randomness.It takes a lot of time to crystallize such an as-deposited amorphousfilm.

[0362] On the other hand, if the range of Vdc in this preferredembodiment is used, the energy of sputter particles emitted from thetarget decreases, and the energy of sputter particles being incident onthe substrate also decreases. As a result, the cooling rate of thesputter particles on the substrate decreases, and the crystallizationholding time during the change from the gaseous phase to the solid phaseincreases, so that fine nuclei are produced. Then, by the presence ofsuch fine nuclei, it can be completely crystallized by irradiating itwith a light beam only once.

[0363] As described above, although Vth<Vdc≦10 Vth is desired in orderto obtain good rapid initializing characteristics, this range is lowerthan the conventional Vdc. However, if the value of Vdc is simply set tobe low, the thin-film deposition rate of the recording layer decreases,and this is not desired for the production efficiency in the sputterprocess. In order to improve the efficiency in the whole process, it isimportant to enhance throughput without increasing the costs in thewhole process or to reduce the costs without damaging throughput in thewhole process so that the initializing process reducing effect exceedsthe decrease of the production efficiency in the sputter process (thedecrease of the thin-film deposition rate) while the value of Vdc is setto be a range capable of obtaining good as-deposited recordingcharacteristics. The efficiency in the whole process can not univocallybe determined since it is a design matter based on the producing timeper disc. As the results of the inventor's experiments (which will bedescribed in detail later), the efficiency in the whole process wasimproved over the whole range of Vth<Vdc≦10 Vth.

[0364] In addition, the improvement of the production efficiency in thesputter process can be also accomplished by improving the sputterprocess itself. For example, there is a method for attempting toincrease the ionic current density (Ii) introduced into the target,i.e., to increase the ion density (Ni) in a negative glow plasma, whilethe range of Vdc is set to be able to obtain good non-initializedfirst-recording characteristics. Assuming that a sputtering yield is γand a target area is St, the sputter emission amount can be expressed byγ (Vdc)·Ii·St, wherein γ (Vdc) shows that γ is a function of Vdc (whichis in proportion to the energy of ions being incident on the target),and γ (Vth)=0. In addition, the relationship between Ii and Ne isIi=e·Ni·vi/4, wherein e denotes the elementary charge amount, and videnotes a random speed of an ion in a negative glow plasma. Therefore,if the Ni is high even if the Vdc is low, it is possible to achieve ahigh thin-film deposition rate. As means for increasing the Ni whileholding a low value of Vdc, there are means for increasing the magneticfield intensity of a magnet for magnetron plasma, means for increasingthe frequency of a plasma exciting power supply, and auxiliary plasmadensity increasing means, such as a hollow cathode electron source, anion source and an inductive coupling plasma producing coil. In addition,as an effective, energy controllable high-density plasma source, thereare ECR plasma and helicon plasma.

[0365] After experiments are repeated using these means, it was foundthat a rapid deposition can be carried out while good non-initializedfirst-recording characteristics are held, when the Ni is adjusted to begreater than 10¹¹ (cm⁻³) The lower limit of the Ni is the condition forobtaining a practical thin-film deposition rate, e.g., a thin-filmdeposition rate of 0.5 nm/sec or higher even if the value of Vdc is alow value of about 2 Vth, and a sufficiently high thin-film depositionrate of about 2 nm/sec when the Vdc is 10 Vth. There is particularly noupper limit of Ni. However, excessive increase of the plasma densitycause the heating of the substrate, the Ni is preferably less than 10¹²(cm⁻³). Since the phase change recording medium producing system in thispreferred embodiment has plasma density increasing means, it is possibleto produce high-density plasma having an Ni of 10¹¹ (cm⁻³) or higher inthe range of Vth<Vdc≦10 Vth.

[0366] The Vdc can be directly read out of a monitor attached to atypical sputtering system. The Vdc can also be monitored by mounting avoltage probe on a target or by observing a voltage waveform by anoscilloscope using a high-frequency high-pressure-proof probe in thecase of the RF discharge. The Ni can be measured by a probe method. Thedetails of the probe method are described in, e.g., “Fundamentals ofPlasma Engineering” by Sinriki Teii (Uchida Rokakuho Shuppan).

[0367] Referring to the accompanying drawings, the examples of thispreferred embodiment will be described below.

[0368]FIG. 23 is a conceptual diagram of a magnetron sputtering systemwhich was used for this preferred embodiment of the present invention.The system shown in FIG. 23 is used mainly for forming a recordinglayer. Other films, such as an interference layer and a reflectivelayer, than the recording layer can be formed by means of the samesputtering system as the conventional sputtering system. Of course, theother films then the recording layer may be formed by means of thesystem of FIG. 23.

[0369] In FIG. 23, reference number 501 denotes a deposition container,502 denoting a sputter source, 521 denoting a sputtering targetconstituting the sputter source 502, 522 denoting a target housingconstituting the sputter source 502, 523 denoting a magnet constitutingthe sputter source 502, 503 denoting a sputter power supply, 531denoting a direct current breaking capacity constituting the sputterpower supply 503, 532 denoting an RF (13.56 MHz) power supplyconstituting the sputter power supply 503, 504 denoting a Vdc monitorsystem, 541 denoting a Vdc monitor constituting the monitor system 504,542 denoting a high-frequency high-pressure-proof probe and anoscilloscope, which constitute the monitor system 504, 505 denoting asubstrate holder, 506 denoting an optical disc substrate, 507 denoting asputter gas supply system, 508 denoting an exhaust system, 509 denotinga plasma probe, 510 denoting a probe circuit, 511 denoting a magnetronplasma, and 512 denoting an inductive coupling coil.

[0370] This example is an example of RF magnetron discharge. Theelectric power supply to the target may be DC. In addition, typicalbipolar discharge, which is not magnetron discharge, may be used forthis preferred embodiment. Of course, a power meter is mounted on thesputter power supply for monitoring a sputter input. In the abovedescribed construction, the Vdc monitor 541 is previously mounted on thesputtering system, and comprises an RF breaking and tuning LC circuitand a direct current voltage (Vdc) monitor. Although the monitor 542comprising the probe and the oscilloscope was particularly required forcarrying out this preferred embodiment, it was provided forconfirmatively measuring the Vdc serving as an essential parameter inthis preferred embodiment. The plasma probe 509 is not mounted on atypical system for sputtering a phase change recording medium. Theplasma probe 509 was mounted for measuring plasma parameters, such as aplasma potential and an ion density, which are related to this preferredembodiment. The probe circuit is a typical probe circuit, and comprisesa voltage applying system to the probe, and a probe current monitoringsystem. The magnetron plasma density (ion density) was herein calculatedfrom an ion saturation current, on which the magnetic field has a smallinfluence. The inductive coupling coil 512 is specially mounted forcarrying out this preferred embodiment, and provided for increasing theplasma density and the ion current density being incident on the target.

[0371] In addition, FIG. 23 shows an example of a so-called staticopposed type sputtering system. This preferred embodiment should not belimited to the positional relationship between the target and thesubstrate, the substrate may be eccentrically arranged with respect tothe target to rotate or revolve around the target. Although the sputtergas is typically argon (Ar) gas, it may be any one of He, Ne, Kr, Xe andmixed gases thereof. In addition, a reactive gas, such as oxygen,nitrogen and hydrogen, may be added if necessary.

[0372] Using the above described system, this preferred embodiment wascarried out by the following procedure.

FIRST EXAMPLE

[0373] In this example, there was examined the relationship between thevalues of Vdc, non-initialized first-recording characteristics andthin-film deposition rates when any methods for enhancing the plasmadensity are not carried out. Using the system shown in FIG. 23, thisexample was carried out by the following procedure.

[0374]FIG. 24 is a conceptual diagram showing the sectional structure ofa recording medium experimentally manufactured in this example.

[0375] In this figure, reference number 506 denotes a polycarbonatesubstrate having tracking grooves. As the substrate 506, anexperimentally manufactured substrate having a diameter of 120 nnm, athickness of 0.6 mm and a track pitch of 0.6 μm was used. In order toexamine the first recording characteristic in a non-initialized state,the structure of a medium film formed on the substrate was a five-layerstructure comprising a gold (Au) semitransparent layer 562 having athickness of 10 nm, a first ZnS—SiO₂ interference layer 563 having athickness of 80 nm, a GeSbTe (2:2:5) recording layer 564 having athickness of 20 nm, a second ZnS—SiO₂ interference layer 565 having athickness of 30 nm, and an Al alloy reflective layer 566 having athickness of 50 nm.

[0376] The medium has a so-called LtoH (lot to high) structure which hasa reflectance of 20% when the state of the recording layer is anamorphous state and a reflectance of 5% when the state of the recordinglayer is a crystalline state. The LtoH structure was adopted in order toenhance the reflectance in a non-initialized state, i.e., anas-deposited amorphous state, to improve the stability of a trackingservo signal. This preferred embodiment may be applied to a mediumhaving a so-called HtoL structure wherein the reflectance in thecrystalline state is higher than the reflectance in the amorphous state.In such a case, the reflectance in the amorphous state should be set soas to sufficiently obtain the stability of servo.

[0377] The sputtering system of FIG. 23 is used for forming a recordinglayer. Other layers than the recording layer may be formed by asputtering system independent of the system of FIG. 23, or by asputtering system connected to the system of FIG. 23. In theundermensioned description, other layers than the recording layer wasformed by a typical sputtering system, i.e., a sputtering system havingthe same construction as that of the system of FIG. 23 except that theVdc confirming oscilloscope system 542, the plasma probe 510 and theinductive coupling coil 512 are not provided. In addition, in order toprevent the surface oxidation of the recording layer after the thin-filmdeposition, the connection type system is used to sequentially formlayers in a vacuum.

[0378] First, after the above described Au semitransparent layer 562 andfirst ZnS—SiO₂ interference layer 563 are formed on the substrate 506,the substrate 506, together with the substrate holder 505, istransported into the deposition container 501 of the sputtering systemof FIG. 23. As described above, in this preferred embodiment, theinductive coupling coil for increasing the plasma density is notoperated when the recording layer is formed. The mass flow controller ofthe sputter gas supply system is adjusted to introduce argon (Ar) gasinto the deposition container at 100 sccm, and the exhaust system isadjusted to hold the gas pressure in the container to be 2 Pa.

[0379] Then, when the RF power supply 503 is turned on to input a powerof P (W) to the sputter source 502, a doughnut-type magnetron plasma isproduced in a space above the GeSbTe target 521, and the Vdc isdisplayed on the Vdc monitor in accordance with the P. The P is changedas a discharge parameter every deposition. With respect to the monitoredvalue of the Vdc, the reading of the monitor 541 attached to the systemwas coincident with the measured value of the confirming voltage proveand oscilloscope system in a range of ±5 V. Therefore, the directreading value of the monitor 541 attached to the sputtering system willbe described below.

[0380] The value obtained by dividing the P by the Vdc is a mean ioncurrent density being incident on the target. Referring to a previouslyexamined thin-film deposition rate, sputter discharge is continuouslycarried out until the GeSbTe recording layer 564 having a thickness of20 nm is deposited on the first interference layer 563. Thereafter, theRF power supply 503 is turned off to interrupt gas. Then, the substrate,on which the recording layer 564 has been deposited, is sequentiallymoved to the deposition chambers for the second interference layer andthe reflective layer 566 to form a phase change disc.

[0381] The disc thus obtained was stuck on a dummy substrate having nomedium film to carry out recording and/or reproducing operations in anas-deposited state. In the formation of the medium film, the depositiontime is often relatively short, and it is difficult to carry out theprobe measurement during the thin-film deposition. Therefore, it isdesired to separately carry out the probe measurement on the sameconditions as those for the deposition to derive the ion density. Theion current density being incident on the target and the ion density inplasma will be described in the next example, and the relationshipbetween the values of Vdc, the first recording characteristics in anon-initialized state, and thin-film deposition rates will be hereindescribed.

[0382]FIG. 25 is a graphs showing the relationship between the values ofVdc/Vth, non-initialized first-recording characteristics, and thin-filmdeposition rates. The value of Vth may be a weighted arithmetical meanof the values of Vth of Ge, Sb and Te, or experimentally derived fromthe thin-film deposition rate and the Vdc (the value of Vth is given bythe interpolated value of the Vdc, by which the thin-film depositionrate is zero). The weighted arithmetical mean of the values of Vth ofGe, Sb and Te described on a sputter data book was substantiallycoincident with the experimentally derived value of Vth to besubstantially 20 V.

[0383] The (Ra−Rc1)/(Ra−Rc) on the axis of ordinates was derived usingthe as-deposited amorphous reflectance (Ra), the reflectance (Rc1) of acrystal space formed by carrying out the first recording operation inthe as-deposited state, and the reflectance (Rc) of a crystal spaceafter overwrite operations were carried out hundred times or more. Sincethe Rc corresponds to the reflectance of a crystal part after theinitial crystallizing step is carried out in accordance with theconventional method, the (Ra−Rc1)/(Ra−Rc) is an index how good crystalspace is formed by the first recording operation in the as-depositedstate. If this value is 100%, it means that the crystal space iscompletely formed from the first recording operation, and if this valueis x %, it means that a part of (100−x) % is not sufficientlycrystallized and the residual amorphous material exists.

[0384] It can be seen from FIG. 25 that it is possible to obtain goodnon-initialized first-recording characteristics by setting Vth<Vdc≦10Vth in accordance with this preferred embodiment. That is, it ispossible to omit the initial crystallizing step from a process forproducing a phase change recording medium. In particular, in a range ofVdc≦8Vth, the (Ra−Rc1)/(Ra−Rc) was 100%, which was a complete value, andthe repeatability thereof was sufficiently high.

[0385] Furthermore, when the Vdc is smaller than 3 Vth, the thin-filmdeposition rate tends to be extremely slow to decrease the film densityto slightly deteriorate the oxidation resistance, so that it is desiredthat 3 Vth≦Vdc.

[0386] The DR on the right axis of ordinates in FIG. 25 denotes thethin-film deposition rate of a recording layer. The thin-film depositionrate obtained by the first example of this preferred embodiment areshown as a “first example” in this figure.

[0387] As can be seen from this figure, the thin-film deposition rate ofthe recording layer decreases as the value of Vdc decreases. Forexample, when the value of Vdc is set to be 2 Vth, the thin-filmdeposition rate of the recording layer is about 0.5 nm/sec. Since atypical value of thin-film deposition rate in the conventional method isabout 2 nm, the thin-film deposition rate is decreased to aboutone-fourth of the conventional typical value when Vdc=2 Vth. However,also in this case, the effect capable of reducing the initializationstep is greater.

[0388] The producing costs will be compared on the conditions that theproducing time per disc is the same as the conventional producing time.The price of the sputtering system is typically about 10 to 20 times aslarge as the price of the initializing system. If the decreased amountof the thin-film deposition rate of the recording layer is replaced withthe number of the increased sputter chambers, one of the increasedsputter chambers increases the price of the whole sputtering system byabout 5 to 10%, so that the decreased amount of the thin-film depositionrate by ¼ can be replaced with the price of four initializing systems.However, ten initializing systems have been conventionally provided foreach sputtering system. On the other hand, according to this preferredembodiment, it is not required to provide ten initializing systems, thenumber of which is far greater than the above described converted number(four) of the initializing systems corresponding to the thin-filmdeposition rate.

[0389] The above described trial calculation is establishedsubstantially over the whole range of Vth<Vdc≦10 Vth in this preferredembodiment. According to the above described trial calculation, thelower limit of Vdc is a value, at which the thin-film deposition rate isdecreased to one-tenth of the conventional rate, and 1.3 Vth in thispreferred embodiment. However, a method for calculating the efficiencyof the whole process depends on the design of the process. The value ofVdc is herein defined so as to be higher than Vth.

[0390] While the Vdc has been controlled mainly by the discharge input(P) in this preferred embodiment, the Vdc slightly varies in accordancewith the kind of gas and the kind of target constituting element. Inaddition, from the results of the inventor's experiments, it wasconfirmed that the DR was in proportion to γ (Vdc)Ii. In this example,the value of Ii was in the range of from 0.4 to 0.8 mA/cm², and thevalue of Ni was in the range of from 2×10¹⁰ to 4×10¹⁰ (cm⁻³), when theVdc/Vth was in the range of from 2 to 10.

SECOND EXAMPLE

[0391]FIG. 26 is a conceptual diagram showing an example of thesectional structure of a phase change recording medium produced in thisexample. In this figure, reference number 571 denotes a disc substrate,572 denoting a lower interference layer, 573 denoting a recording layer,574 denoting an upper interference layer, and 575 denoting a reflectivelayer.

[0392] In this example, layers other than the recording layer 573 wereformed by a technique using purer, a typical magnet (1T class), a powersupply of 13.56 MHz and no feedback, without the sputtering system ofFIG. 23. In addition, all of the layers were formed in a vacuum.

[0393] Furthermore, while the four-layer structure has been adopted asthe layer structure of a typical phase change recording medium in thispreferred embodiment, this preferred embodiment should not be limited tothe layer structure of the film. For example, this preferred embodimentmay be widely applied to the structure shown in FIG. 4 on pp.66-67 ofTechnical Digest of Joint-MORIS/ISOM 1997, the structure shown in FIG. 1on pp.74-75 thereof, the structure shown in FIG. 1(a) on pp.23-24 ofPost Deadline Paper, Technical Digest thereof, the structure shown inFIG. 1 on pp.104-109 of Digest of Tenth Phase change Recording WorkshopSymposium, and the structure disclosed in Japanese Patent Laid-Open No.10-226173.

[0394] A pre-formatted disc of polycarbonate is typically used as a discsubstrate. The typical diameter of the substrate is 64 mm, 80 mm, 120mm, 135 mm, 300 mm or the like, and the typical thickness of thesubstrate is 0.6 mm or 1.2 mm. In this example, a DVD-RAM format discsubstrate was used. Although the upper and lower interference layers aretypically formed of ZnS-20% SiO₂, these layers may be suitably formed ofa transparent dielectric material, such as Ta—O, Si—O, Si—N, Al—N, Ti—O,B—N and Al—O.

[0395] The recording layer is typically formed of Ge—Sb—Te orAg—In—Sb—Te. In this example, the recording layer was formed ofGe₂Sb₂Te₅.

[0396] The reflective layer may be formed of a high reflectancematerial, such as Al alloys, Au, Cu, Ag and Ti—N. In this example, thereflective layer was formed of an Al—Mo alloy.

[0397] The thickness of the lower interference layer 572 was 120 nm, andthe thickness of the recording layer 573 was 20 nm. In addition, thethickness of the upper interference layer 574 was 15 nm, and thethickness of the reflective layer 575 was 100 nm. In opticalcalculations, with respect to a light beam having a wavelength of 650nm, the reflectance of the amorphous part is 5% and the reflectance ofthe crystal part is 20%.

[0398] Then, a procedure for producing a phase change recording mediumhaving the structure of FIG. 26 will be described. The substrate 571 wasloaded on the sputtering system, and the sputtering system was evacuatedto form the lower interference layer 572 on the conventional conditions.Thereafter, the substrate was transported into the sputtering systemhaving the structure of FIG. 23, and the recording layer 573 was formedby the following procedure.

[0399] That is, the thin-film deposition container 501 has beenpreviously evacuated, and the substrate, together with the substrateholder 505, is transported in a vacuum from the chamber for forming thelower interference layer 572. An Ar—Kr mixed gas is introduced at 200sccm from the gas supply system 507. Then, while the pressure in thethin-film deposition container 501 is maintained to be 2 Pa, the pulsemodulated RF power supply 532 is inputted to produce a doughnut-typemagnetron plasma 511 in a space above the GeSbTe target 521.

[0400] A cathode fall is formed between the negative glow plasma and thetarget, so that the target has a potential of −Vdc with respect to theground potential. Among ions in the plasma, ions diffused to the cathodeare accelerated toward the target to impact on the target at an energyof substantially Vdc, so that target constituting elements are emittedby sputtering. Since the energy during the sputter emission issubstantially in proportion to the energy of ions being incident, i.e.,the Vdc, the energy of sputter particles in this preferred embodiment isset to be lower than that in the conventional method.

[0401] After the recording layer was deposited as described above, theupper interference layer 574 and the reflective layer 575 weresequentially laminated again by the conventional sputtering method, andthe disc was taken out to atmosphere.

[0402] Also in this example, a large number of optical recording mediawere experimentally manufactured using Vdc as a parameter, and therecording characteristic in the as-deposited state was evaluated. As aresult, the relationship between the values of Vdc/Vth, thenon-initialized first-recording characteristics and the thin-filmdeposition rates was substantially the same as that shown in FIG. 25,and it was possible to obtain good non-initialized first-recordingcharacteristics in a range of Vth<Vdc≦10 Vth.

[0403] Furthermore, as will be described in detail later, by producing ahigh-density plasma using the above described various plasma densityincreasing means, the amount of sputter emission can be a high value ofseveral nm/sec, which is substantially the same as that in theconventional method, even at a lower Vdc than that in the conventionalmethod.

THIRD EXAMPLE

[0404] The above described first and second examples show basic forms ofthis preferred embodiment mainly in view of the relationship between thevalues of Vdc and the non-initialized first-recording characteristics.In this third example, a plasma density increasing means is furtherprovided in order to enhance the thin-film deposition rate while thevalue of Vdc is set to be in a desired range on the first recordingcharacteristic in a non-initialized state.

[0405] In this example, while the construction of the sputtering systemused for depositing a recording layer is the same as that in FIG. 23, itis attempted to increase the magnetic field intensity of the magnetronplasma producing magnet 523 or to use the inductive coupling plasmaproducing coil 512, or to use both. Although the value of Vdc varies bythe increase of the magnetic field intensity and the production of theinductive coupling plasma, the discharge input (P) to the sputter sourceis also adjusted so that the value of Vdc is in the range of 2Vth≦Vdc≦10 Vth, in which good non-initialized first-recordingcharacteristics are basically obtained.

[0406] The procedure for carrying out the third example by means of thesystem having the structure of FIG. 23 should be the followingimprovement of the above described procedure in the first example. Thatis, the magnet 523 is changed from a member for producing a magneticfield of about 1 to 1.5 kG to a member for producing a magnetic field ofabout 2 to 2.5 kG. Alternatively, a power is supplied to the inductivecoupling plasma coil 512 at the same time that a power is applied to thesputter source. Alternatively, both may be carried out.

[0407] In order to increase the magnetic field intensity, theconstituent material of the magnet may be changed to a high Bs material,or the design of the magnetic circuit may be improved. The inductivecoupling plasma coil is designed to produce a so-called inductivecoupling plasma (ICP) used for a semiconductor process system or thelike. In the inductive coupling plasma coil, a Cu coil or an SiO₂ coatedCu coil is provided in the vicinity of the target in the thin-filmdeposition container, and an RF power is inputted from the outside toproduce the ICP.

[0408] In this example, the value of Vdc was monitored, and the iondensity (Ni) was measured by means of the plasma probe 509, so that thefirst recording characteristics in the non-initialized state and thethin-film deposition rates were examined by the same method as that inthe first example. The first recording characteristics in thenon-initialized state had a good value of Vth<Vdc≦10 Vth even in a casewhere an auxiliary plasma density increasing means, such as an inductivecoupling plasma producing means, are used, and a magnetron fieldintensity is increased. This means that not only the first recordingcharacteristics in the non-initialized state, but also the finestructure of the as-sputtered film, do not depend on the plasma densityduring the thin-film deposition of the recording layer, and arecontrolled by the value of Vdc, i.e., non only the energy of ions beingincident on the target, but also the energy of sputter particles emittedfrom the target.

[0409] The thin-film deposition rate is in proportion to λ (Vdc)·Iisimilar to the first example. As described above, Ii can be expressed asIi=e·Ni·vi/4 using the ion density (Ni) in the plasma and the randomspeed (vi) of ions in the plasma. Since vi is about 5×10. Since the viis about 5×10⁵ (cm/sec) in view of an ion temperature of about 1000 K,the Ii can be presumed from the Ni in the prove measurement, so that thethin-film deposition rate can be presumed. In the above described firstexample, i.e., in a case where the plasma density was not particularlyincreased, the value of Ni was in the range of from 2×10¹⁰ to 4×10¹⁰(cm⁻³) and the value of Ii was in the range of from 0.4 to 0.8 mA/cm²,in the range of Vdc/Vth of 1 to 10, in which good non-initializedfirst-recording characteristics were obtained. On the other hand, inthis example, the value of Ni was a high value of 10¹¹ (cm⁻³), and thevalue of Ii was a high value of 2 mA/cm², in the range of Vdc/Vth of 2to 10.

[0410] The thin-film deposition rates (DR) obtained in this example areexpressed by a “third example” in FIG. 25. As can be seen clearly fromthis figure, when a high-density plasma is produced in accordance withthis example, the thin-film deposition rate of the recording layerexceeds the conventional typical value, so that the efficiency of thewhole process can be remarkably improved.

[0411] In the above described example, the magnetron field intensityincreasing means has been used as the plasma density increasing means,and the inductive coupling coil has been used as the auxiliary plasmagenerating means. However, there may be used various methods, such as amethod for providing a hollow cathode type electron source, and a methodfor providing an ion source (a differential exhaust system may be alsoprovided when the operation pressure is lower than that in the sputterchamber), and a method for utilizing an ECR plasma or a helicon plasma.

FOURTH EXAMPLE

[0412] In this example, an Ar-10% Kr mixed gas was used. By adding Krhaving a large mass number, it is possible to enhance the sputteremission efficiency even if a low ion energy is used. In order to lowerthe value of Vdc, a gas having a small mass number is preferably used.Therefore, when a rare gas is used, He, Ne, Kr and Xe are preferablymixed at a suitable ratio.

[0413] Although FIG. 23 shows an example of a typical magnetron sputtersource, the magnet of FIG. 23 may be arranged on the reverse surface ofthe target, or the magnet may be arranged on the target on the same sideas that of the substrate. Alternatively, a typical bipolar sputtersource (non-magnetron type) or an ECR sputter source may be used inplace of the magnetron sputter source. Moreover, the above describedauxiliary plasma producing means for enhancing the plasma density ispreferably added. In this example, a NdFeB magnet having a strongmagnetic field source (>2T class) was adopted as the plasma densityincreasing means to enhance the efficiency for capturing electrons in anegative glow. The sputter power supply may be DC or RF, and an AC powersupply having a suitable frequency may be used in place of a typical RFhaving a frequency of 13.56 MHz. In addition, a pulse modulated plasmamay be used to increase the density.

[0414] In this example, an RF power supply having a frequency of 13.56MHz pulse-modulated by 10 kHz was used. By pulse-modulating by 10 kHz,the bipolar diffusion loss of ions and electrons from the negative glowis decreased, and the plasma density is increased. In this preferredembodiment, since its point is the control of Vdc, the Vdc in dischargeis preferably timely monitored to control the sputter power supply by afeedback circuit so that the Vdc is always a predetermined value, inorder to control the variation in Vdc due to disturbance. By thefeedback system, the fluctuations in Vdc and Ni can be controlled to beless than ±1%.

[0415] Other constructions may be the same as those of the conventionalsputtering system. Comparing the sputtering plasma in this example withthe conventional sputtering plasma, the value of Ni was 3×10¹⁰ (cm⁻³) orless in the conventional system wherein pure Ar, a typical magnet (−1Tclass), a power supply of 13.56 MHz and no feedback were used, whereasthe Ni was 10¹¹ (cm⁻²) to be greatly improved to realize a highthin-film deposition rate in this example.

[0416] In the conventional system, the value of Ni was about 3×10¹⁰(cm⁻³) at the most even if a high power of about 2 kW was inputted,although the value of Ni depends on the input power. In addition, in thecase of no feedback, the fluctuation in Ni in discharge was in the rangeof about ±20%.

FIFTH EXAMPLE

[0417] The rapid initialization characteristic substantially depends onthe value of Vdc, and does not depends on the number of ions beingincident on the target, i.e., the value of Ni. Therefore, this preferredembodiment may be applied particularly without increasing the plasmadensity. An example wherein this preferred embodiment is applied on thecondition that the plasma density is less than 10¹¹ (cm⁻³), e.g.,Ni=4×10¹⁰ (cm⁻³), will be described below.

[0418] The medium to be used is the same as that in the above describedfirst example. The recording layer was deposited by a technique usingpure Ar, a typical magnet (−1T class), a power supply of 13.56 MHz andno feedback.

[0419] On the condition that Vdc=10 Vth, it was possible to obtain anas-deposited recording characteristic, which was far superior to that ofa recording medium produced on the conventional condition (typicallyVdc>13 Vth) and which was able to be practically used. At this time, thethin-film deposition rate of the recording layer was maintained to be arate of about 90% of the conventional typical value, so that it wasclearly possible to improve the efficiency of the whole process.

[0420] In addition, when the value of Vdc was set to be a low value of 2Vth, the thin-film deposition rate of the recording layer was decreasedto about one-eighth of the conventional typical value. Although thisincreases the costs by eight initializing systems, the effect forreducing the initialization step corresponds to ten initializingsystems, so that the efficiency of the whole process is improved.

[0421] When the value of Vdc is too close to the value of Vth, thedecreased amount of the thin-film deposition rate is more remarkablethan the acceleration due to the removal of the initialization step.This balance point may be the condition that the thin-film depositionrate is decreased to one-tenth of the conventional rate. This conditionis approximately Vdc=1.7 Vth in this example.

SIXTH EXAMPLE

[0422] Then, the material of the recording layer was changed fromGe—Sb—Te to Ag₈In₁₃Sb₄₉Te₃₀ (at %), and this preferred embodiment wasapplied by the same technique as those in the above described examplesusing Ge—Sb—Te.

[0423] As a result, similar to the comparative examples wherein therecording layer was produced in accordance with the conventional method,and similar to the case of Ge—Sb—Te, it was possible to obtain goodas-deposited recording characteristics in the whole range of Vth<Vdc≦10Vth, and it was confirmed that the initialization step reducing effectwas greater than the thin-film deposition rate reducing rate to improvethe efficiency of the whole process.

[0424] While the examples of this preferred embodiment have beendescribed, this preferred embodiment should not be limited thereto.

[0425] That is, the film material and thickness of the respectivelayers, and the methods and conditions for depositing films other thanthe recording film should not be limited, except that the sputtercondition for the recording layer is important for the application ofthis preferred embodiment. For example, the material of the recordinglayer may be selected from chalcogen metal compounds, such as Ge—Sb—Teand Ag—In—Sb—Te, and materials suitably containing a very small amountof Cr, V, N or the like, in place of the above described materials.

[0426] In addition, in the case of a five-layer film structure, thesemitransparent layer may be formed of silver (Ag), copper (Cu), silicon(Si) or a film having a structure wherein fine metal particles aredispersed in a dielectric matrix, in place of Au. In addition, thematerial of the interference layer may be suitably selected fromdielectric film materials, such as Ta₂O₅, Si₃N₄, SiO₂, Al₂O₃ and AlN, inplace of ZnS—SiO₂, and the material of the recording layer may besuitably selected from chalcogen film materials, such as InSbTe,AgInSbTe and GeTeSe, in place of GeSbTe. Moreover, the material of thereflective layer may be suitably selected from Al alloy film materials,such as AlCr and AlTi, in place of AlMo.

[0427] Moreover, while the optical disc has been used as an opticalrecording medium in the above described example, this preferredembodiment should not be limited thereto. For example, this preferredembodiment may be applied to any one of various optical recording media,such as an optical recording card, to obtain the same advantages asthose of this preferred embodiment.

[0428] According to this preferred embodiment, it is possible to carryout a recording operation at a high CNR immediately from an as-depositedstate, so that it is possible to remove an initial crystallizing stepfrom a process for producing a phase change recording medium. As aresult, it is possible to reduce the producing costs and to cause phasechange recording media to spread widely.

[0429] (Sixth Preferred Embodiment)

[0430] The sixth preferred embodiment of the present invention will bedescribed below. In this preferred embodiment, there is provided amethod and system for producing a phase change recording medium, whereina recording layer on a substrate is heated on unique conditions andwherein an initial crystallizing step is omitted.

[0431] In order to the above described object, a method for producing aphase change recording medium according to this preferred embodiment isa method for producing a phase change recording medium having asubstrate and a recording film deposited thereon, wherein fine nucleiare produced in the recording film by raising the temperature of therecording film to a higher temperature than room temperature while thetemperature of the substrate is set to be less than its thermaldeformation temperature, while or after the recording film is depositedon the substrate.

[0432] On the other hand, a system for producing a phase changerecording medium according to this preferred embodiment is a system forproducing a phase change recording medium having a substrate and arecording film deposited thereon, which comprises heating means forraising the temperature of the recording film to a higher temperaturethan room temperature while setting the temperature of the substrate toits thermal deformation temperature, to produce fine nuclei in therecording film.

[0433] Alternatively, a system for producing a phase change recordingmedium according to this preferred embodiment is a system for producinga phase change recording medium having a substrate and a recording filmdeposited thereon, which comprises means for depositing the recordingfilm on the substrate, and heating means for raising the temperature ofthe recording film to a higher temperature than room temperature whilesetting the temperature of the substrate to its thermal deformationtemperature, to produce fine nuclei in the recording film.

[0434] In this preferred embodiment, the heating means is preferably aninfrared ray lamp.

[0435] The system further comprises a substrate holder for supportingthe substrate, the substrate holder having a contact portion with thesubstrate, the contact portion being made of a material which does notsubstantially absorb an infrared ray emitted by heating the lamp.

[0436] The basic concept of this preferred embodiment is that while orimmediately after the phase change recording layer is deposited, thetemperature of the recording layer is raised to crystallize therecording layer or to produce fine nuclei in an amorphous network.Conventionally, when a practical resin substrate was used as a substratefor an optical disc, it was impossible to raise the temperature of arecording layer to a higher temperature than room temperature during andafter the thin-film deposition since the thermal deformation temperatureof the substrate is lower than the crystallizing temperature of therecording layer.

[0437] The inventor found that if the temperature of a recording layeris raised to a lower temperature than the thermal deformationtemperature of a substrate during or immediately after the thin-filmdeposition, it is possible to form nuclei in an amorphous networkalthough it is not possible to completely crystallize the recordinglayer, and made the first through third inventions.

[0438] Moreover, the inventor found that if an infrared ray lamp systemcapable of rapidly heating and cooling is adopted, it is possible toraise only the temperature of a recording film having a very smallthermal capacity in a time to prevent a thermal load to be applied to asubstrate having a large thermal capacity, and it is not only possibleto form a structure that fine nuclei are scattered in an amorphousnetwork, but it is also possible to form a recording film in acrystalline state, and made the fourth invention.

[0439] In addition, the inventor found that if a material, which doesnot absorb lamp rays, is used as the material of the contact portion ofthe substrate holder with the substrate in the application of the fourthinvention, the thermal deformation of the substrate does not occur evenif the lamp heating is carried out for a sufficiently long time in orderto crystallize the recording film, and made the fifth invention.

[0440] The reason why the initial crystallizing step can be reduced toaccomplish the object of this preferred embodiment by the recording filmformed in this preferred embodiment, i.e., by the recording film havingthe structure wherein the fine nuclei are scattered in the amorphousnetwork, will be briefly described. As described above, a recording markis made of an amorphous material in view of the principle of the phasechange recording, and the amorphous part of the recording mark iscrystallized by the irradiation with erasing beams for about tensnanoseconds without annealing for a long time, so that it seems that itis not hindered from carrying out a recording operation on anas-deposited amorphous material without passing through the initialcrystallizing step. However, it is not actually possible to carry out arecording operation on the as-deposited amorphous material since it isnot possible to form a crystal space thereon. On the other hand, oncethe initial crystallization is carried out, even if the amorphous markis formed by the irradiation with light, the amorphous mark is rapidlycrystallized by the irradiation with erasing beams.

[0441] The significant difference between the as-deposited amorphousmaterial and the amorphous material formed by the optical recordingoperation can not be found by the typical X-ray diffraction or electronbeam diffraction. The inventor observed the details of the as-depositedamorphous material and the amorphous part formed by the opticalrecording operation using a high resolution electron microscope. As aresult, no regular structure was particularly observed in theas-deposited amorphous material. On the other hand, a short-range orderhaving a size of about sub-nanometer to about several nm was observed inthe amorphous material formed by the optical recording operation. Thatis, the inventor found that it was possible to rapidly crystallize theamorphous state including the short-range order by promoting thecrystallization by the short-range order although it was not possible torapidly crystallize the as-deposited amorphous part having highrandomness and no order.

[0442] This preferred embodiment was made on the basis of this finding.That is, according to this preferred embodiment, it is possible toeffectively heating the recording layer without damaging the substrateof the phase change recording medium, to form the short-range ordercapable of causing the rapid crystallization. As a result, it is notrequired to carry out the initial crystallization, so that it ispossible to reduce the producing costs and to cause phase changerecording media to spread widely.

[0443] In Arrhenius handling, assuming that a crystallizing constant isα, a ratio (X) of a crystal region in amorphous material isx=1−exp(−αt), wherein t is time. In addition, α can be expressed by α=νexp(−Ea/kT) using a frequency factor (ν) for crystallization and anactivation energy (Ea) for crystallization, wherein k is the Boltzmann'sconstant and T is an absolute temperature. Therefore, it is found thatcrystallization depends on both temperature and time to proceed and thatcrystallization gradually proceeds even at room temperature in a longtime of tens years.

[0444] In this preferred embodiment, the heating time to produce finenuclei or the heating time for crystallization corresponds to a tact forproducing a phase change recording medium, and is several seconds toseveral minutes. In this time range, fine nuclei are produced at a lowertemperature than a crystallizing temperature obtained by a so-called DSCmeasurement. This can be also analogized by the fact thatcrystallization is easily carried out by heating at a temperature ofabout 200 to 300° C. even if the heating is carried out for tensnanoseconds to about hundred nanoseconds by the irradiation with laserbeams.

[0445] The relationship between the heating time and heatingtemperature, which was experimentally derived by the inventor, is asfollows. That is, when a recording layer having a structure, whereinfine nuclei are scattered in an amorphous material, (which is opticallysubstantially an amorphous state), is formed, the heating temperature ispreferably 80 to 90° C. in the case of a heating time of severalminutes, 100 to 110° C. in the case of a short heating time of severalseconds to about ten seconds, and 120° C. or higher in the case of aheating time of one second or less.

[0446] On the other hand, when the recording layer is intended to becrystallized, the heating temperature is preferably 110 to 120° C. inthe case of a heating time of several minutes, 130 to 140° C. in thecase of a heating time of ten and several seconds, and 150° C. or higherin the case of a heating time of one second or less.

[0447] Furthermore, since the migration of sputter particles on thesubstrate is promoted when heating is carried out during the thin-filmdeposition, it is preferably to carry out heating during the thin-filmdeposition since a short-range order is more easily formed than whenheating is carried out after the thin-film deposition, i.e., aftermigration is completed to be solidified, to promote the solid phasediffusion.

[0448] The material of the substrate of the phase change recordingmedium will be described below.

[0449] As the substrate of the phase change recording medium, apolycarbonate substrate having a thermal deformation temperature ofabout 120° C. is typically used. As substrate materials other thanpolycarbonate, a polymethyl methacrylate (PMMA) or an amorphouspolyolefin (PO) may be used as the material of a substrate of an opticaldisc for optical characteristics. The thermal deformation temperature ofPMMA is 85° C. in the case of an injection molded article, and about100° C. in the case of a cast article. In addition, the thermaldeformation temperature of PO is about 130° C. The present invention iseffectively applied to a PMMA or PO substrate in place of thepolycarbonate substrate.

[0450] Moreover, the present invention may also be applied to a newoptical disc substrate material which is studied to be adopted infuture. When only the recording layer is selectively heated by theinfrared ray lamp in this preferred embodiment, it is effective even ifthe thermal deformation temperature of the substrate is lower than thecrystallizing temperature of the recording layer (the standard of whichis the DSC measured value), and the infrared ray lamp has the merits ofbeing easy to be produced and of being capable of more rapidly heatingand cooling.

[0451] Moreover, if the contact portion of the substrate holder with thesubstrate is made of a material which does not substantially absorb lamprays emitted from the infrared ray lamp, it is possible to selectivelyheat the recording layer, so that it is possible to inhibit the thermaldeformation of the substrate.

[0452] Referring to the accompanying drawings, the examples of thispreferred embodiment will be described below.

[0453]FIG. 27 is a conceptual diagram showing an example of a phasechange recording film forming system for use in this preferredembodiment. In this figure, reference number 601 denotes a thin-filmdeposition container, 602 denoting a substrate holder, 603 denoting aresistance heater, 604 a heater power supply, 605 denoting an opticaldisc substrate, 606 denoting a phase change recording film, 607 denotinga sputter source, 608 denoting a GsSbTe target, 609 denoting a sputterpower supply, 610 denoting a gas supply system, 611 denoting an exhaustsystem, and 612 denoting a substrate heating infrared ray lamp.

[0454] In the first example of this preferred embodiment, the resistanceheater 603 is mainly used for raising the temperature of the recordingfilm in FIG. 27. In the second example of this preferred embodiment, theinfrared ray lamp is used for raising the temperature of the recordingfilm. However, both the resistance heater and the infrared ray lamp maybe used for raising the temperature of the recording film. In addition,the thin-film deposition container 601 may be a separate batch typecontainer, a batch type container provided in a loading/unloadingchamber, or one chamber of a plurality of thin-film depositioncontainers connected in an inline type or single wafer processingsystem.

FIRST EXAMPLE

[0455] In this example, only the resistance heater 603 is used forraising the temperature of the phase change recording film. Using theabove described construction of FIG. 27, this preferred embodiment wasapplied in accordance with the following procedure. The thin-filmdeposition container is one chamber of a single wafer processingsputtering system. The polycarbonate substrate 605 is mounted in a frontchamber, which is evacuated, and then the substrate 605 is delivered toeach of the thin-film deposition containers. Before the recording film606 is deposited, a first interference film of ZnS—SiO₂ is formed on thesubstrate 5 by a predetermined method. The substrate 5 having the firstinterference film is delivered to the evacuated thin-film depositioncontainer 601. The substrate 605 and the holder 602 may be movedtogether or separately.

[0456] After the heater 603 is energized to hold the temperature of thesubstrate to, e.g., 85° C., e.g., Ar gas is supplied from the gas supplysystem 610 at 200 sccm, and the gas pressure in the container is heldto, e.g., 0.25 Pa. Then, when the sputter power supply 609 is turned onto input an RF power to the sputter source 607, a magnetron plasma isproduced in the vicinity of the target 608. In a cathode fall formedbetween the target and the plasma, Ar ions in the plasma are acceleratedtoward the target to impact on the surface of the target at an energy ofhundreds eV. Then, particles of Ge, Sb and Te constituting the targetare sputter-emitted, and a GeSbTe film is deposited on the substrate 605facing the target. Thereafter, a second interference film, e.g.,ZnS—SiO₂ film, and a reflective film, e.g., an Al alloy film, aresequentially deposited on the substrate delivered to another thin-filmdeposition container, and the substrate is ejected to atmosphere. It isnot required to heat a part of the recording layer other than thethin-film deposited part, at a temperature zone less than the thermaldeformation temperature of the substrate.

[0457] In this example, the layer structure of the medium was a typicalHtoL (high to low) structure wherein the reflectance of a crystallinematerial is higher than the reflectance of an amorphous material, andthe thickness of each of the layers was adjusted so that the reflectanceof the amorphous material was 10% and the reflectance of the crystallinematerial was 35%. These set values were applied in order to verify theeffects of this preferred embodiment. This means that the values ofreflectance were set so as to be able to obtain stable servo signalswhen the as-deposited state is an amorphous state. However, thispreferred embodiment may be applied to the LtoH structure in place ofthe structure having the above described values of reflectance.

[0458] The phase change recording medium formed by the above describedprocedure will be hereinafter referred to as a “medium 1 in the firstexample”. That is, the medium 1 in the first example is obtained byheating to a temperature zone less than the thermal deformationtemperature of the substrate using the resistance heater 603 during thethin-film deposition of the recording layer. Thus, a plurality of media1 in the first example were formed while the substrate temperaturevaries during the thin-film deposition.

[0459] Then, by the same procedure as the above described procedure, thesubstrate is not heated during the thin-film deposition of the recordingfilm, and the heater 603 is energized to heat the substrate for severalminutes after the thin-film deposition of the recording layer, so that aphase change recording medium having the same layer structure as that ofthe above described medium. This will be hereinafter referred to as a“medium 2 in the first example”. Also in this case, a plurality of discswere experimentally manufactured using an annealing temperature afterthe thin-film deposition of the recording film as a parameter.

[0460] Then, as a comparative example, a disc having the same layerconstruction as that of the above described disc was produced withoutheating the substrate both during and after the thin-film deposition ofthe recording film. This will be hereinafter referred to as a“comparative medium”.

[0461] The “medium 1 in the first example”, the “medium 2 in the firstexample” and the “comparative medium”, which were obtained in accordancewith the above described procedure, were evaluated by the followingprocedure. First, after the disc ejected from the sputtering system wasstuck on a counter blank substrate, the disc was set in a discevaluating system without carrying out the initial crystallizing step.Then, a signal was recorded at a linear velocity of 8.2 m/sec and arecording bit length of 0.28 μm/bit.

[0462] In addition, noise level was measured in order to examine thedamage to the substrate due to the heating of the substrate. After thereflectance of the disc was examined before the recording operation, allof the media 1 and 2 in the first example and the comparative media hada value of about 10%, so that it was suggested that the initial state,i.e., the as-deposited state, was macroscopically an amorphous state.

[0463]FIG. 28 is a graph showing the evaluated results of the discs. Inthis figure, white circles denote the evaluated results of thecomparative medium, black circles denote the evaluated results of themedium 2 in the first example, and black triangles denote the evaluatedresults of the medium 1 in the first example. With respect to the noiselevels, no increase of noise was observed up to about 80° C. by heatingboth during and after the thin-film deposition, and the increase ofnoise was observed from about 100° C. due to the thermal deformation ofgrooves of the substrate or the increase of the “warp” of the substrate.When the temperature exceeded 120° C. being the thermal deformationtemperature of polycarbonate although it was not plotted in the figure,the “warp” was remarkably increased. Thus, the alignment to the countersubstrate was not good, or it was not possible to obtain stable servosignals even if the alignment was good, so that it was not possible toevaluate the discs.

[0464] The CNR was clearly high from the as-deposited first-recordingoperation when the recording film is heated during or after thethin-film deposition in accordance with this preferred embodiment. Thetemperature zone for heating the recording film was preferably in therange of from 70 to 110° C., more preferably in the range of from 75 to105° C., most preferably about 85° C. Comparing the heating during thethin-film deposition (medium 1) with the heating after the thin-filmdeposition (medium 2), the CNR was better in the case of the heatingduring the thin-film deposition. It is considered that the reason forthis is that although the phase change recording film formed by thispreferred embodiment has the structure wherein the fine nuclei arescattered in the amorphous network as described above, the short-rangeorder is more easily formed when heating is carried out to promotemigration while sputter particles migrate on the surface of thesubstrate during the thin-film deposition, than when heating is carriedout to promote the diffusion of a solid phase after the thin-filmdeposition, i.e., after the migration of sputter particles is completedto solidify the sputter particles.

SECOND EXAMPLE

[0465] In this example, the infrared ray lamp 612 was used for heatingthe recording film without the need of the resistance heater 603 in theconstruction of FIG. 27. Lamp heating was not continuously carried out.In order to avoid the temperature rise of the substrate holder, “cycleheating” operations for turning the lamp off after the temperaturereaches a set temperature and for cooling the temperature to about 50°C. to heat again to the set temperature were repeated three times. Bythis method, the time for the recording layer to be heated to the settemperature was several seconds to about ten seconds.

[0466] In addition, in this example, a substrate holder 602 of a typicalSUS member shown in FIG. 27, and a substrate holder of a memberabsorbing substantially no infrared ray were used.

[0467]FIG. 29 is a conceptual diagram showing an example of a substrateholder which does not absorb lamp rays. In this figure, reference number621 denotes a structural member of SUS, 622 denoting a member of amaterial which is transparent to infrared rays and which has lowabsorptivity, 623 denoting an air gap for preventing the heat of thestructural member 621, which absorbs infrared rays to be heated, from betransmitted to the transparent member 622, 605 denoting a disc substratewhich contacts the transparent member 622 to be mounted thereon, and 606denoting a predetermined film provided on the substrate 605. Thetransparent member 622 may be made of a glass member having goodworkability, such as polycrystalline glass which can be suitable forcutting and sharing process, quartz, alumina, or the like.

[0468] Similar to the first example, the temperature of the substratewas set so as to locally work the peripheral portion of the substrate,and the recording layer of the substrate was arranged on the oppositeside to the target. In the case of lamp heating, the thermocouple itselfdirectly absorbs lamp rays to be heated so as not to be able tocorrectly measure temperature. Therefore, a thin-film for substantiallyreflecting all of lamp rays was provided so as to face the target, inwhich the thermocouple was inserted. Thus, it is possible to correctlymeasure the temperature of the recording film.

[0469] In order to carry out evaluation, the reflectance of therecording film was first measured. As a result, it was revealed that thevalue of reflectance was about 30% and the recording film wascrystallized when the lamp heating temperature was 140° C. or higher.When the heating temperature was 130° C. or less, the value ofreflectance was about 10%, and the state of the recording film was anamorphous state. When the lamp heating temperature was in the range offrom 130 to 140° C., the reflectance reflected in partialcrystallization was obtained.

[0470]FIG. 30 is a graph showing the evaluated results of the values ofCNR and noise levels. In this figure, black circles denote the resultswhen the substrate holder 602 of FIG. 27 was used, and white circlesdenote the results when the substrate holder shown in FIG. 29 was used.In this example, the noise level tends to rise when the heatingtemperature is 130° C. or higher. Comparing with data shown in FIG. 28,i.e., with the case where heating is carried out by the resistanceheater, is can be seen that the heating temperature for raising thenoise level in this example is shifted to a higher temperature.

[0471] In the resistance heating, the substrate holder is heated to heatthe substrate to heat the recording film. Thus, the thermal deformationof the substrate occurs from a low temperature. on the other hand, sincethe recording film is directly heated in this example, the thermaldeformation of the substrate is remarkably suppressed when heating andcooling are cyclically carried out even if the SUS substrate holder ofFIG. 27 is used. Moreover, it can be seen that the thermal deformationof the substrate is remarkably suppressed when only the recording filmis selectively heated using the holder of FIG. 29 and when the substrateis not heated (a polycarbonate substrate typically used for an opticaldisc is transparent to infrared rays).

[0472] When the holder of FIG. 27 was used, the increase of the noiselevel and the decrease of the first CNR due to the deformation of thesubstrate by heating at 120° C. or higher were observed. When the holderof FIG. 29 was used, the noise level was not increased by heating at170° C. Similar to the resistance heating shown in FIG. 28, the firstCNR was a high value unless the noise level was produced at atemperature of 80° C. or higher, so that the effect of this preferredembodiment was verified.

[0473] While the examples of this preferred embodiment have beendescribed, this preferred embodiment should not be limited thereto.

[0474] For example, while the four-layer structure, wherein theZnS—SiO₂/GeSbTe/ZnS—SiO₂/Al alloy film was deposited on the substrate bysputtering, has been used as the film structure of Rc <Ra of the mediumin the above described example, a five-layer structure, wherein an Ausemitransparent film is inserted in the four-layer structure, may beused.

[0475] In addition, the film material and thickness of the respectivelayers, and the methods and conditions for depositing films other thanthe recording film should not be limited, except that the condition forheating the recording layer is important for the application of thispreferred embodiment. For example, the material of the recording layermay be selected from chalcogen metal compounds, such as Ge—Sb—Te andAg—In—Sb—Te, and materials suitably containing a very small amount ofCr, V, N or the like, in place of the above described materials.

[0476] In addition, in the case of the five-layer film structure, thesemitransparent layer may be formed of silver (Ag), copper (Cu), silicon(Si) or a film having a structure wherein fine metal particles aredispersed in a dielectric matrix, in place of Au. In addition, thematerial of the interference layer may be suitably selected fromdielectric film materials, such as Ta₂O₅, Si₃N₄, SiO₂, Al₂O₃ and AlN, inplace of ZnS—SiO₂, and the material of the recording layer may besuitably selected from chalcogen film materials, such as InSbTe,AgInSbTe and GeTeSe, in place of GeSbTe. Moreover, the material of thereflective layer may be suitably selected from Al alloy film materials,such as AlCr and AlTi, in place of AlMo.

[0477] Moreover, while the optical disc has been used as an opticalrecording medium in the above described example, this preferredembodiment should not be limited thereto. For example, this preferredembodiment may be applied to any one of various phase change opticalrecording media, such as an optical recording card, to obtain the sameadvantages as those of this preferred embodiment.

[0478] As described in detail above, according to this preferredembodiment, it is possible to carry out a recording operation at a highCNR immediately from an as-deposited state, so that it is possible toremove an initial crystallizing step from a process for producing aphase change recording medium. As a result, it is possible to reduce theproducing costs and to cause phase change recording media to spreadwidely.

[0479] (Seventh Preferred Embodiment)

[0480] The seventh preferred embodiment of the present invention will bedescribed below.

[0481] In this preferred embodiment, there is provided a method andsystem for producing a phase change recording medium, which can increasethe storage capacity and stably acquire address signals and sesignals byproviding a plurality of recording layers and which can prevent theproductivity form being lowered by the initial crystallizing step.

[0482] As techniques for improving the recording density of a phasechange medium, there are techniques for decreasing the wavelength of alight source, for increasing the NA of an objective lens, for applying asuper resolution thin-film and so forth.

[0483] As means for improving the storage capacity without the need ofthe improvement of the recording density, a single-sided double-layerdisc is proposed. The single-sided double-layer disc is designed torecord and reproduce data on two recording layers, which are apart fromthe same plane of light beam incidence by about tens μm, by onlyadjusting the focal point of the light beam, and has substantially thesame performance as a single-sided single-layer disc havingsubstantially a double recording density when being viewed from theuser, since it is not required to turn the disc over. As a reproductiononly DVD, there is known a single-sided double-layer disc which is knownas a common name DVD-9. However, it has been considered that since thetransmittance of a rewritable DVD is insufficient by one recordinglayer, light does not sufficiently reach the recording layer arranged atthe bottom with respect to the incident side of light beams, so that itis difficult to record and reproduce data.

[0484] However, in ISOM (International Symposium Optical Memory) '98,Technical Digest, pp. 144-145 (Th—N-05), it has been suggested that itis possible to form a single-sided double-layer even in the case of arewritable phase change medium. The points of this technique are thatthe transmittance of a first recording layer part is increased to about50% so that light sufficiently reaches a second recording layer partarranged at the bottom when the first recording layer part and thesecond recording layer part are arranged in that order from the incidentside of light beams, that the reflectance of the second recording layerpart is set to be higher, i.e., the transmittance thereof is set to belower, in order to maintain the balance of servo signals andregenerative signals from the first and second recording layer parts,and that the absorptivity Ac of the crystal part is set to be higherthan the absorptivity Aa of the amorphous part in both of the first andsecond recording layer parts in order to reduce overwrite jitters.

[0485] In order to satisfy the above described setting, the firstrecording layer part has a three-layer construction which has aso-called HtoL structure, wherein the reflectance Rc of the crystal partis higher than the reflectance Ra of the amorphous part, and which hasno reflective film, and the second recording layer part has a five-layerconstruction which has a thin Au semitransparent film below a so-calledLtoH structure, wherein the reflectance Rc of the crystal part is lowerthan the reflectance Ra of the amorphous part, a thin Au semitransparentfilm underlying the LtoH structure, and a thin Al—Cr reflective filmabove the LtoH structure.

[0486] In this construction, with respect to the reflectance of eachrecording layer part viewed from the incident side of light beams, thereflectance of the first recording layer part is 9% of that of thecrystal part and 2% of that of the amorphous part, and the reflectanceof the second recording layer part is about 3% of that of the crystalpart and about 9% of that of the amorphous part. Therefore, if thesingle-sided double-layer phase change medium is initial-crystallized inaccordance with the conventional producing process, the initialreflectance of the address part and data part is about 9% in the firstrecording layer and about 3% in the second recording layer. This initialreflectance is far lower than, e.g., 15% to 25% of a single-sidedsingle-layer DVD-RAM standard. At the initial reflectance of the firstrecording layer, it is possible to reproduce address signals and servosignals of the data part somehow if the reproducing power is increased.However, the reflectance of the second recording part is too low, sothat it is difficult to reproduce both of address signals and servosignals.

[0487] In addition, the common problem of single-sided double-layermedia, which are not limited to the above described rewritable media, isthat the initial crystallizing step is complicated. That is, if each ofthe first and second recording layer parts is initial-crystallized, itis required to carry out double steps to obstruct the productivity andproducing costs.

[0488] This preferred embodiment has been made in view of the abovedescribed problems on conventional single-sided double-layer media, andit is an object of this preferred embodiment to provide a method andsystem for producing a phase change recording medium, which canreproduce good address signals and servo signals and which can solve theproblem that the initial crystallizing step is complicated to damage theproductivity.

[0489] In order to the above described object, according to thispreferred embodiment, there is provided a phase change recording medium,which comprises: a first recording layer part for causing a phase changebetween a crystalline state and an amorphous state by irradiation withlight; a separation layer formed on the first recording layer part; anda second recording layer part, formed on the separation layer, forcausing a phase change between the crystalline state and the amorphousstate by irradiation with light, wherein the state of an address part ofat least one of the first and second recording layer parts is anamorphous state having substantially the same randomness as that of anamorphous recording mark of a data part.

[0490] Alternatively, there is provided a phase change recording medium,which comprises: a first recording layer part including a firstsubstrate, a first lower interference layer formed on the firstsubstrate, a first recording layer, formed on the first lowerinterference layer, for causing a phase change between a crystallinestate and an amorphous state by irradiation with light, and a firstupper interference layer formed on the first recording layer; aseparation layer formed on the first upper interference layer; and asecond recording layer part including a second lower interference layerformed on the separation layer, a second recording layer, formed on thesecond lower interference layer, for causing a phase change between thecrystalline state and the amorphous state by irradiation with light, asecond upper interference layer formed on the second recording layer,and a reflective layer formed on the second upper interference layer,wherein the state of an address part of at least one of the first andsecond recording layer parts is an amorphous state having substantiallythe same randomness as that of an amorphous recording mark of a datapart.

[0491] Moreover, there is provided a method for producing a phase changerecording medium, which comprises a first substrate, a first recordinglayer part, formed on the first substrate, for causing a phase changebetween a crystalline state and an amorphous state by irradiation withlight, a separation layer formed on the first recording layer part, anda second recording layer part, formed on the separation layer, forcausing a phase change between the crystalline state and the amorphousstate by irradiation with light, wherein the initial crystallization ofthe first recording layer part and the initial crystallization of thesecond recording layer part are substantially simultaneously carriedout.

[0492] In addition, according to this preferred embodiment, there isprovided a system for producing a phase change recording medium, whichcomprises: a first holding part for holding a first substrate, on whicha first recording layer part for causing a phase change between acrystalline state and an amorphous state by irradiation with light isdeposited; a second holding part for holding a second substrate, onwhich a second recording layer part for causing a phase change betweenthe crystalline state and the amorphous state by irradiation with lightis deposited; a light irradiation part for irradiating the first andsecond recording layer parts with initial crystallizing light beams forinitial-crystallizing the first and second recording layer parts; and anoptical system for condensing the initial crystallizing light beamspassing through the first recording layer part to irradiate the secondrecording layer part with the condensed initial crystallizing lightbeams.

[0493] The preferred examples of a phase change recording medium in thispreferred embodiment will be described below.

[0494] (1) The thermal conductivity of the address part having theamorphous state having substantially the same randomness as that of theamorphous recording marks of the data part is in the range of from 0.8to 6 W/mK.

[0495] (2) In the crystal space of the data part of the recording layerhaving the address part having a thermal conductivity of 0.8 to 6 W/m,there is the maximum value at each of at least two different grainsizes, in the distribution of the number of crystal particles withrespect to the grain sizes.

[0496] (3) The amorphous recording marks of the data part in (2) arealigned in a narrower crystal space than the width of a track, each ofthe aligned marks having a smaller width than the crystal space, and thewidth of the track being narrower than a laser spot diameter determinedby an operating wavelength and the numerical aperture of an objectivelens.

[0497] The preferred examples of a method for producing a phase changerecording medium in this preferred embodiment will be described below.

[0498] (1) The initial crystallization is carried out by irradiationwith the initial crystallizing light beams, and part of the initialcrystallizing light beams for irradiating the first recording layer partis used for initial-crystallizing the second recording layer part.

[0499] (2) The method comprises the steps of depositing the firstrecording layer part on the first substrate, depositing the secondrecording layer part on the second substrate, and sticking the first andsecond substrates together via the separation layer, on the sides of thefirst and second substrates on which the first and second recordinglayers are deposited, after the initial crystallizing step.

[0500] First, a phase change recording medium in this preferredembodiment will be described below.

[0501] When the initial state is an as-deposited state, it is generallydifficult to carry out a recording operation (formation of a crystalspace), so that it is not possible to obtain significant regenerativesignals unless the same track is repeatedly overwritten. However, if thetechnique described in detail in, e.g., the third preferred embodiment,is used, it is possible to obtain significant regenerative signals fromthe first recording operation even in the as-deposited amorphous state.

[0502] If the gist of the technique proposed in the third preferredembodiment is briefly described, it is possible to obtain significantregenerative signals from the first overwrite operation without the needof the initial crystallizing step by causing the as-deposited amorphousstate to approach the amorphous recording mark formed by the opticalrecording operation using laser beams.

[0503] In this preferred embodiment, if the above described technique inthe third preferred embodiment is applied to a recording layer havingthe LtoH structure of single-sided double-layer structures, thequalities of address signals and servo signals are greatly improved, andit is not required to carry out the initial crystallizing step, so thatthe productivity is improved.

[0504] By the way, since any data do not written on the address part ofthe medium by the user, the as-deposited state is maintained even afterthe overwrite operation. It has been revealed by the inventor's studythat the thermal conductivity of the amorphous state in the address partof at least one of the first and second recording layer parts in thispreferred embodiment is in the range of from 0.8 to 6 W/mK.

[0505] It has also been revealed that the crystal space of the data parthas the maximum value in each of at least two different grain particlesin the distribution of the number of crystal grains with respect tograin sizes, and the amorphous recording marks are aligned in thecrystal space, each of the aligned marks having a width which is notgreater than that of the crystal space, the width of a track beingnarrower than a laser spot diameter determined by an operatingwavelength and the numerical aperture of an objective lens.

[0506] When a single-sided double-layer phase change recording mediumdisclosed in the above described ISOM (International Symposium onOptical Memory) '98, Technical Digest, pp.144-145 (Th—N-05) is appliedto this preferred embodiment, only the application to the secondrecording layer part is effective, and the first recording layer partpasses through the initial crystallizing step similar to theconventional method. However, this preferred embodiment should not belimited thereto. For example, there can be applied a producing method,wherein when the first recording layer part uses a medium having theLtoH structure having high transmittance, the initial state of the firstrecording layer part can also be an amorphous state, and when the firstrecording layer part uses the LtoH structure having high transmittanceand when the second recording layer part uses the HtoL structure havinghigh transmittance, only the initial state of the first recording layerpart can be an amorphous state, and the second recording layer partpasses through the initial crystallizing step similar to theconventional method.

[0507] In either case, the stability of address signals and servosignals of the recording layer part having the LtoH structure is notonly ensured. When only one of the first and second recording layerparts has the LtoH structure, the initial crystallizing step is also thesame as that of the conventional single-sided single-layer disc, andwhen both the first and second recording layer parts have the LtoHstructure, the producing process is more simplified than theconventional process, so that the productivity it improved to reduce theproducing costs.

[0508] Although the recording layer having the as-deposited amorphousstate in this preferred embodiment is particularly effective in therecording layer having the LtoH structure, it may be applied to therecording layer having the HtoL structure.

[0509] A method and system for producing a phase change recording mediumin this preferred embodiment will be described below. In this preferredembodiment, the producing method is characterized by the initialcrystallization of a phase change recording medium. In particular, inthe case of the HtoL structure, there is an advantage in that if theinitial crystallization is carried out, the reflectance of the initialstate of the address part and data part is enhanced so that thestability of servo is good.

[0510] First, the first recording layer part is deposited on the firstsubstrate, and the second recording layer part is separately depositedon the second substrate. The first and second substrates are held in thefirst and second holding parts in the producing system in this preferredembodiment.

[0511] The producing system for carrying out the initial crystallizationhas a light irradiation part for irradiating with initial crystallizinglight beams, and an optical system for condensing the initialcrystallizing light beams passing through the first recording layer partto irradiate the second recording layer part with the condensed initialcrystallizing light beams. Although the light beams emitted form thelight irradiation part are first incident on the first recording layerpart to initial-crystallize the first recording layer part, part of thebeams passes through the first recording layer part to diverge. Thediverging beams are condensed again by the optical system, and thesecond recording layer part is irradiated with the condensed beams, sothat the second recording layer part is initial-crystallizedsubstantially at the same time that the first recording layer part isinitial-crystallized.

[0512] After the initial crystallization is completed, the firstrecording layer part deposited side of the first substrate is stuck onthe second recording layer part deposited side of the second substratevia the separation layer to accomplish a phase change recording medium.

[0513] By adopting such an initial crystallizing step, it is enough tocarry out only one initial crystallizing step, so that the producingprocess is simplified and the productivity is not damaged.

[0514] Furthermore, a producing method and system in this preferredembodiment may be applied to both the HtoL structure and the LtoHstructure.

[0515] Referring to the accompanying drawings, the examples of thispreferred embodiment will be described below.

FIRST AND SECOND EXAMPLES

[0516]FIGS. 31 and 32 are schematic sectional views showing two examplesof a phase change optical disc serving as a phase change recordingmedium in the first and second examples in this preferred embodiment. InFIGS. 31 and 32, the same reference numbers are used for members havingthe same functions.

[0517] In FIGS. 31 and 32, the recording and/or reproducing light beamsare radiated from the lower side of the figures. In both FIGS. 31 and32, reference number 701 denotes a first recording layer part, 702denoting a second recording layer part, 731 denoting a first substrate,732 denoting a second substrate, and 704 denoting a separation layer forseparating the first recording layer part from the second recordinglayer part.

[0518] The film structures of the first recording layer part 701 and thesecond recording layer part 702 in FIG. 31 are different from those inFIG. 32.

[0519] In FIG. 31, the films constituting the first recording layer part701 include a first lower interference layer 711, a first recordinglayer 712 and an upper interference layer 713, in that order from thelight beam incident side, and the films constituting the secondrecording layer part 702 include a semitransparent layer 721, a secondlower interference layer 722, a second recording layer 723, a secondupper interference layer 724 and a reflective layer 725, in that orderfrom the light beam incident side.

[0520] On the other hand, in FIG. 32, the films constituting the firstrecording layer part 701 include a first lower interference layercomprising three layers of a lower interference layer 714, a lowerinterference layer 715 and a lower interference layer 716, a firstrecording layer 717 and a first upper interference layer 718, and thefilms constituting the second recording layer part 702 include a secondlower interference layer 726, a second recording layer 727, a secondupper interference layer 728 and a reflective layer 729.

[0521] In the disc of FIG. 31, the first recording layer part 701 hasthe HtoL structure, and the second recording layer part 702 has the LtoHstructure. In the disc of FIG. 32, the first recording layer part 701has the LtoH structure, and the second recording layer part 702 has theHtoL structure.

[0522] The discs of FIGS. 31 and 32 were produced by, e.g., thefollowing procedure.

[0523] The first substrate 731 of polycarbonate having tracking groovesand a pre-pit header part and having a thickness of, e.g., 0.58 mm, andthe second substrate 732 of polycarbonate having a thickness of, e.g.,0.6 mm, can be produced by a typical optical disc substrate producingprocess.

[0524] The formation of films on the first substrate 731 is carried outby, e.g., the following procedure.

[0525] In the disc of FIG. 31, the first substrate 731 is mounted in asubstrate holder of a magnetron sputtering system, and evacuation iscarried out. Then, in a sputter chamber wherein a ZnS—SiO₂ target ismounted in a sputter source, the target is sputtered in, e.g., anatmosphere of Ar gas plasma at 0.4 Pa, to form the first lowerinterference layer 711 having a mean thickness of, e.g., 80 nm.Subsequently, in a sputter chamber having a GeSbTe target, the target issputtered in, e.g., an atmosphere of Ar gas plasma at 0.4 Pa, to formthe first recording layer 712 having a mean thickness of 7 nm. Then, inthe sputter chamber wherein the ZnS—SiO₂ target is mounted in thesputter source, the target is sputtered in, e.g., an atmosphere of Argas plasma at 0.4 Pa, to form the first upper interference layer 713having a mean thickness of 30 nm to eject the substrate from thesputtering system.

[0526] Subsequently, the second substrate 732 is mounted in a sputteringsystem to form the reflective layer 725 of Au having a mean thickness of10 nm in, e.g., an atmosphere of Ar gas plasma at 0.4 Pa. Subsequently,the second upper interference layer 724 of ZnS—SiO₂ having a meanthickness of 25 nm is formed in, e.g., Ar gas plasma at 0.1 Pa. Then,the recording layer 723 of GeSbTe having a mean thickness of 12 nm isformed in, e.g., an atmosphere of Kr gas plasma at 4 Pa. Subsequently,the second lower interference layer 722 of ZnS—SiO₂ having a meanthickness of 85 nm is formed in, e.g., an atmosphere of Ar gas plasma at0.1 Pa. Finally, the semitransparent layer 721 of Au having a meanthickness of 8 nm is formed in, e.g., an atmosphere of Ar gas plasma at0.4 Pa. Then, the substrate is ejected from the sputtering system.

[0527] The first and second substrates 731 and 732, on which the filmshave been thus formed, are stuck together so that the first and secondrecording layer parts 701 and 702 formed on the first and secondsubstrates 731 and 732, using a transparent adhesive sheet or aUV-curing adhesive layer at a spacing of 40 μm to form the separationlayer 704 to obtain the disc of FIG. 31 in the first example.

[0528] The disc of FIG. 32 was produced by, e.g., the followingprocedure.

[0529] The first substrate 731 is mounted in a substrate holder of amagnetron sputtering system, and evacuation is carried out. Then, in asputter chamber wherein a ZnS—SiO₂ target is mounted in a sputtersource, the target is sputtered in, e.g., an atmosphere of Ar gas plasmaat 0.4 Pa, to form the lower interference layer 714 having a meanthickness of, e.g., 60 nm. Subsequently, in a sputter chamber wherein aSiO₂ target is mounted, the target is sputtered in, e.g., an atmosphereof Ar gas plasma at 0.4 Pa, to form the lower interference layer 715having a mean thickness of 100 nm. Then, in the sputter chamber whereinthe ZnS—SiO₂ target is mounted in the sputter source, the target issputtered in, e.g., an atmosphere of Ar gas plasma at 0.1 Pa, to formthe lower interference layer 716 having a mean thickness of 60 nm. Thus,the first lower interference layer comprising the lower interferencelayer 714, the lower interference layer 715 and the lower interferencelayer 716 is produced. Subsequently, in a sputter chamber wherein aGeSbTe target is mounted, the target is sputtered in, e.g., anatmosphere of Kr gas plasma at 8 Pa, to form the second recording layer717 having a mean thickness of 8 nm. Then, in a sputter chamber whereina ZnS—SiO₂ target is mounted in a sputter source, the target issputtered in, e.g., an atmosphere of Ar gas plasma at 0.1 Pa, to formthe first upper interference layer 718 having a mean thickness of 40 nmto eject the substrate from the sputtering system.

[0530] On the second substrate 732, the reflective layer 729 of Auhaving a mean thickness of 10 nm is formed in, e.g., an atmosphere of Argas plasma at 0.4 Pa. Subsequently, the second upper interference layer728 of ZnS—SiO₂ having a mean thickness of 25 nm is formed in, e.g., Argas plasma at 0.4 Pa. Then, the second recording layer 727 of GeSbTehaving a mean thickness of 12 nm is formed in, e.g., an atmosphere of Argas plasma at 0.4 Pa. Subsequently, the second lower interference layer722 of ZnS—SiO₂ having a mean thickness of 85 nm is formed in, e.g., anatmosphere of Ar gas plasma at 0.4 Pa to eject the substrate from thesputtering system.

[0531] The first and second substrates 731 and 732, on which the filmshave been thus formed, are stuck together so that the first and secondrecording layer parts 701 and 702 formed on the first and secondsubstrates 731 and 732, using a transparent adhesive sheet or aUV-curing adhesive layer at a spacing of 40 μm to form the separationlayer 704 to obtain the disc of FIG. 32 in the second example.

PROCESS IN FIRST AND SECOND EXAMPLES

[0532] The above described thin-film deposition process is characterizedby a process for depositing the upper and lower interference layers,between which the recording layer having the LtoH structure and therecording layer are sandwiched. As described above, the high pressuresputtering of the recording layer with the heavy rare gas and the lowpressure sputtering of the upper and lower interference layers areimportant in order to ensure the recording characteristic from the firstrecording operation while the initial state of the recording film ismaintained to be an as-deposited amorphous state. These thin-filmdeposition processes are basically characterized by the control of thecooling rate when the sputter particles are cooled on the surface of thesubstrate.

[0533] With respect to a technique for recording on the as-depositedamorphous state without the need of the initializing step, a basic ideawill be described below. This amorphous state is a substantiallyamorphous state, and means an optically amorphous state. For example,this state means a state wherein the light reflectance, which isimportant as the characteristic of the recording medium, is closer tothe reflectance of the amorphous mark than the reflectance of thecrystal space.

[0534] The recording layer for use in a phase change recording operationis typically deposited by a sputtering method, and the state of therecording layer is an amorphous state immediately after the depositionthereof. The sputtering method is a technique for producing apredetermined film by allowing gaseous-phase sputter particles, whichare sputter-emitted from the surface of a target by a high-energy Ar ionbombardment, to arrive at the surface of a substrate at random tomigrate the surface in a liquid-phase random state, and thereafter, byallowing the state of the phase to be a solid phase serving as a film.

[0535] The transition rate of a sputter particle from a gaseous phase toa solid phase is typically about 10¹² K/sec. That is, it is guessed thatthe time required for a random state of several eV (tens of thousands K)to be changed to a solid phase at room temperature is about 10nanoseconds, and that the time required to pass through a temperaturezone between a melting point and a crystallizing temperature is about 1nanosecond at the most.

[0536] On the other hand, the time to crystallize a GeSbTe or InSbTerecording film is tens nanoseconds. The condition for allowing a film tobe crystallized is that the time to crystallize the film is shorter thanthe crystallization holding time, so that the state of the recordinglayer immediately after the sputter deposition is an amorphous state.

[0537] This amorphous state immediately after the thin-film depositionis different from the amorphous state formed by the optical recordingoperation. Because the cooling rate during the optical recordingoperation is typically about 10¹⁰ K/sec, which is smaller than thecooling rate in a sputter deposition process by about two digits,although it depends on the linear velocity and the layer structure ofthe disc. If the amorphous state immediately after the sputterdeposition has the same quality as that of the amorphous state formed bythe optical recording operation, it is possible to carry out therecording and/or reproducing operation without passing through theinitial crystallizing step. Since the amorphous state immediately afterthe sputter deposition is actually different from the amorphous stateformed by the optical recording operation due to the difference incooling rate, it is difficult to carry out a recording operation fromthe first time unless the initial crystallizing step is carried out, inthe case of both the disc of Rc>Ra and the disc of Rc<Ra.

[0538] A method for forming a phase change recording medium of Rc<Ra inthe first and second examples, i.e., a phase change recording mediumhaving a LtoH disc, is a method capable of recording from the firstrecording operation without passing through the initial crystallizingstep.

[0539] A concrete method for achieving this method is to lower thecooling rate of sputter particles in the sputter process to cause theamorphous state immediately after the thin-film deposition toapproximate to the amorphous state formed by the optical recordingoperation, or to apply a compressive stress to the recording layerimmediately after the sputter deposition to allow the recording layer tobe easily crystallized, or a combination thereof.

[0540] In order to cause the amorphous state immediately aftersputtering to approximate to the amorphous state formed by the opticalrecording operation, there may be used a method for increasing theenergy of sputter particles being incident on the substrate, or a methodfor increasing the time for surface migration. According to theinventor's study, the difference in cooling rates from a molten state isreflected in randomness. That is, as the cooling rate is higher,randomness is higher to form a completely amorphous state. However, whenthe cooling rate is low, there is provide a structure wherein finenuclei are scattered and which microscopically has a short-range orderalthough it is macroscopically random. That is, the cooling rate of theamorphous state formed during the optical recording operation is low,such a fine crystal structure is provided. Also in the as-depositedamorphous state, if it is possible to form such a fine crystalstructure, i.e., if it is possible to form substantially the samerandomness as that of the amorphous recording marks formed by theoptical recording operation, it is possible to carry out a recordingoperation from the first time. The reason for this is that fine nucleiserve as seeds for crystal growth, so that crystallization sufficientlyproceeds unlike a pure amorphous state having no seed for crystalgrowth.

[0541] Furthermore, the expression “substantially the same randomness”means randomness that particularly significant crystal peaks are notdetected by a typical X-ray diffraction for evaluating crystallinity andcrystal structure, and the size distribution of fine nuclei providing aregular atomic arrangement of less than several nm, typically about 0.5to 4 nm, is defined in the range of about ±50%. The “size distribution”is expressed by the mean grain size and grain size dispersion. Inaddition, the “grain size” means a mean of the longest diameter andshortest diameter of a crystal grain.

[0542] As a method for examining the distribution of fine nuclei, thereis adopted, e.g., a method for sampling a recording layer part of a discfrom the surface of the disc at random to observe a region several μmsquare by a high resolution electron microscope.

[0543] In addition, the “short-range order” means the order of regularatomic arrangement existing in a region of less than several nm,typically about 0.5 to 4 nm.

[0544] As a method for forming such a randomness, there is firstmentioned a method for decreasing energy of sputter particles beingincident on a substrate. In this method, there are a plurality ofmethods. Each of these methods will be described below.

[0545] The first method is a method for causing the relationship betweena voltage Vdc applied to a target and a sputter threshold voltage Vth ofa target constituting element to meet the condition of Vth<Vdc≦10 Vthwhen a recording layer is deposited on a substrate by sputtering. Bymeeting this condition, a fine crystal structure is also formed in anas-deposited amorphous state.

[0546] The detailed description of this point is omitted since it hasbeen described in detail with respect to the above described fifthpreferred embodiment.

[0547] The second method will be described below. While or after arecording layer is deposited on a substrate, if the recording film isheated to a higher temperature than room temperature while thetemperature of the substrate is maintained to a lower temperature thanthe thermal deformation temperature thereof, the surface migration timeincreases, so that fine nuclei are produced in the recording film.

[0548] This point has been described in detail in the above describedsixth preferred embodiment.

EVALUATION OF FIRST AND SECOND EXAMPLES

[0549] Referring to FIGS. 31 and 32 again, descriptions will continue.The phase change optical discs shown in FIG. 31 and 32 were evaluated byan optical disc tester after the HtoL structure part was treated by atypical initial crystallizing process while the as-deposited amorphousstate of the LtoH structure part is maintained. The evaluationconditions include a laser wavelength of 650 nm, a NA of an objectivelens of 0.6, a linear velocity of 8.2 m/s, a shortest bit length of 0.31μm/bit, and a track pitch of 0.6 μm as described above.

[0550] In both discs of FIGS. 31 and 32, the reflectance of tworecording layer parts in an unrecorded state was about 10%. Althoughthis reflectance is slightly smaller than that of the current DVD-RAMstandard, practically sufficient address signals and servo signals wereobtained by setting regenerative signals to be slightly high or thelike. In addition, the jitter value after an overwrite operation was agood value, about 10%, in both the two recording layers of the discs ofFIGS. 31 and 32.

COMPARATIVE EXAMPLES WITH FIRST AND SECOND EXAMPLES

[0551] For comparison, media obtained by forming the LtoH parts of FIGS.31 and 32 by a typical sputtering process were prepared, and the sameevaluation as the above described evaluation was carried out. Although ahigh reflectance of about 10% was obtained from the LtoH parts of bothmedia, the jitter value after the overwrite operations tens times was15% which was not a practical value. It is considered that the recordinglayer formed by a typical sputter process does not have fine nucleihaving a high randomness and a short-range order, so that it isdifficult to form a crystal space and it is not possible to obtain asignificant jitter characteristic by carrying out overwrite operationsonly tens times. In addition, for comparison, the LtoH part formed bythe above described typical sputter process was initial-crystallized. Asa result, the initial reflectance was a very low value of about 2 to 3,so that the quality of address signals and the stability of servo wereconsiderably deteriorated.

MODIFIED EXAMPLES OF FIRST AND SECOND EXAMPLES

[0552] In modified examples of a phase change optical disc which has aLtoH recording layer formed by the sputtering process in the first andsecond examples and which is simply produced at lower producing costs,the LtoH recording layer parts of FIGS. 31 and 32 have a two-layerlaminated structure. That is, both the first and second recording layersare most preferably produced by the sputtering process in the first andsecond examples to carry out a recording operation on both layers fromthe as-deposited amorphous state without passing through the initialcrystallizing step.

[0553] The disc having the LtoH recording layer parts as the first andsecond recording layer parts can be obtained by, e.g., combining theLtoH recording layer parts of FIGS. 31 and 32. In this case, after thefirst and second recording layer parts are deposited, both the recordinglayer parts can be actually operated without passing through the initialcrystallizing step.

[0554] The experimentally manufactured discs having the LtoH recordinglayer parts as both the recording layer parts were evaluated at a laserwavelength of 650 nm, a NA of an objective lens of 0.6, a linearvelocity of 8.2 m/s and a shortest bit length of 0.31 μm/bit. The trackpitch is 0.6 μm similar to the above described case. The reflectance oftwo recording layer parts in an unrecorded state was about 10%. Althoughthis reflectance is slightly smaller than that of the current DVD-RAMstandard, practically sufficient address signals and servo signals wereobtained by setting regenerative signals to be slightly high or thelike. In addition, the jitter value after an overwrite operation was agood value, about 10%, in both the two recording layers.

THIRD EXAMPLE

[0555] The third example of this preferred embodiment will be describedbelow. This example relates to a method for initial-crystallizing aphase change optical disc having recording layer parts having the HtoLstructure as first and second recording layer parts, and a producingsystem for carrying out the initial crystallization. Although thisexample is technically different from the above described examples,there are the same advantages in that it is possible to improve thestability of address signals and servo signals and it is possible toprevent the producing process from being complicated to reduce theproducing costs.

[0556]FIG. 33 is a schematic sectional view of a producing system forinitial-crystallizing a third example of a phase change optical disc inthis preferred embodiment.

[0557] In FIG. 33, reference number 701 denotes a first recording layerpart deposited on a first substrate 731, 702 denoting a second recordinglayer part deposited on a second substrate 732, 705 denoting a spindlemotor serving as first and second holding parts for holding the firstand second substrates for rotating a disc, 706 denoting a motor shaft ofthe spindle motor 705, 707 denoting a light irradiation part forirradiating with an initial crystallizing light beam, and 708 denoting aconvergent lens as an optical system. The light irradiation part 707 isusually called an initializing system. The first and second holdingparts may be separately provided.

[0558] The first recording layer part 701 and the second recording layerpart 702 preferably have the HtoL structure. For example, the firstrecording layer part 701 may be the first recording layer part 701 ofFIG. 31, and the second recording layer part 702 may be the secondrecording layer part 702 of FIG. 32. The first substrate 731 and thesecond substrate 732 may be the same as those in the first and secondexamples.

[0559] Using the system of FIG. 33, a disc may be produced by thefollowing procedure. First, the first substrate 731 having the firstrecording layer part 701 produced by the typical sputtering process, andthe second substrate 732 having the second recording layer part 702produced by the typical sputtering process are coaxially mounted on theshaft 706 of the spindle motor 705 of the system of FIG. 33.

[0560] Then, the spindle motor 705 is driven to rotate the first andsecond substrates 731 and 732 at a linear velocity of, e.g., about 2m/s, and the light irradiation part 707 is driven to irradiate the firstrecording layer part 701 with initial crystallizing light beamsextending in a radial direction of the disc. About 50% of theirradiation beam passes through the first recording layer part 701 todiverge. This diverging beam has not been utilized in the conventionalinitial crystallizing step. According to this example, the beam passingthrough the first recording layer part 701 to diverge is condensed againby the convergent lens 708 provided between the first and secondsubstrates 731 and 732 to irradiate the second recording layer part 702with the condensed beam.

[0561] The initial crystallizing light beam is an elliptical broad beamhaving a length of hundreds μm in a radial direction of the disc and alength of several μm in a circumferential direction of the disc.Therefore, even if the first and second substrates 731 and 732 arewarped, it is possible to apply sufficiently high initial crystallizingenergy.

[0562] The focal length of the convergent lens 708 is set to be abouthalf of the distance between the first substrate 731 and the secondsubstrate 732. Although the shape of the lens may be a typicalpoint-symmetrical convex lens shape, it is preferably a semicylindricalshape wherein the curvature of an initial crystallizing light beam in adirection of the major axis is smaller than the curvature thereof in adirection of the minor axis, in order to cause the beam profile on thefirst recording layer part 701 to be coincident with the beam profile onthe second recording layer part 702.

[0563] If necessary, the convergent lens 708 may be moved vertically bymeans of a voice coil motor or the like.

[0564] After both of the recording layer parts are thus substantiallysimultaneously initial-crystallized by the same light beam, if both arestuck together by the above described typical sticking process, the discin this example is obtained. Of course, the reflectance of the discsurface and the recording characteristics are the same as those of theHtoL recording layer parts in the above described first and secondexamples.

MODIFIED EXAMPLE OF THE THIRD EXAMPLE

[0565] Another method for substantially simultaneouslyinitial-crystallizing both of the recording layer parts is a method forproviding two light heads having different focal points serving as thelight irradiation part in the system. The first recording layer part isirradiated with an initial crystallizing light beam emitted from thefirst head to be initial-crystallized, and the second recording layerpart is irradiated with an initial crystallizing light beam emitted fromthe second head to be initial-crystallized.

[0566] While the examples of this preferred embodiment have beendescribed, this preferred embodiment should not be limited thereto.

[0567] For example, while the material of the recording layer has beenGsSbTe in the above described examples, the material of the recordinglayer may be selected from InSbTe, AgInSbTe, GeTeSe, SnSeTe, GeSeSn andInSeTl, in place of GeSbTe. In addition, a very small amount of at leastone selected from the group consisting of Co, Pt, Pd, Au, Ag, Ir, Nb,Ta, V, W, Ti, Cr and Zr may be added to the above described materials toobtain good characteristics as a recording layer. Moreover, a very smallamount of a reducing gas, such as nitrogen, may be added thereto.

[0568] The mean thickness to of the first recording layer is preferablyin the range of from 5 nm to 20 nm in order to ensure the minimum lightabsorption and the light transmittance of the second recording layer andsubsequent recording layers. In addition, the mean thickness Tn of thesecond recording layer and subsequent recording layers preferably meetsthe relationship of T_(n)≦T_(n−n) in order to maintain the sameadvantages.

[0569] In addition, the separation layer is preferably made of atransparent material having an extinction coefficient k of 0.1 or lesswith respect to a light source wavelength, in order to cause the energyloss of the light beam to be minimum. Such materials may be suitablyselected from resin materials, such as polymethyl methacrylate andpolycarbonate, oxides, such as SiO₂, Al₂O₃, TaO, V₂O₅, CaO, ZrO₂, Pb₂O₃,SnO₂, CoO, CuO, Cu₂O, AgO, ZnO and Fe₂O₃, nitrides such as Si₃N₄, SiONand SiAlON, and fluorides, such as MgF₂ and CaF₂, in addition toUV-curing resins. In order to separate the recording layer at a greaterdepth than the focal depth of the light beam, the mean thickness of theseparation layer must be 10 μm or more. Therefore, an application typeresin material suitable for the production of a film having suchthickness is more preferably selected. If necessary, these materials maybe used as a mixture or lamination. In addition, if the mean thicknessof the separation layer is too great, the transmittance of theseparation layer and the focal depth with respect to two or morerecording layers are not desired, so that the mean thickness of theseparation layer is preferably 50 μm or less.

[0570] In addition, the semitransparent film may be formed of Ag, Cu, Sior a film having a structure wherein fine metal particles are dispersedin a dielectric matrix. Similar to the above described Au, in the caseof Ag or Cu, the mean thickness of the semitransparent film ispreferably in the range of from 3 to 20 nm, more preferably in the rangeof from 5 to 15 nm, with respect to an operating wavelength of, e.g.,650 nm. In the case of Si, the mean thickness of the semitransparentfilm is preferably in the range of from 10 to 80 nm, more preferably inthe range of from 30 to 60 nm. In the case of the film having thestructure wherein the fine metal particles are dispersed in thedielectric matrix, the mean thickness of the semitransparent film ispreferably set to be (5-20)/q (nm) in the range of 0.25≦q≦0.75 assumingthat the content of the fine metal particles by volume in the film is q.By adopting the above described mean thickness, it is possible toimprove the efficiency for light utilization of the second recordinglayer part, so that it is possible to carry out a high sensitiverecording operation even by a low intensity of light passing through thefirst recording layer part. In addition, since the ratio of the lightabsorption coefficient of the crystal space in the second recordinglayer part to the light absorption coefficient of the amorphousrecording mark can be set to be in the range of from 1 to 1.5, theoverwrite jitter can be effectively reduced. In addition, since thereflectance of the second recording layer part can be set to be high,there is also an advantage in that the quantity of light passing throughthe first recording layer part to be reflected on the second recordinglayer part is increased.

[0571] The reflective layer may be formed of, e.g., AlTi, TiN, AlMo,AlCu, Ag, Cu, Pt, Pd or Ir. The mean thickness of the reflective layeris preferably in the range of from 20 to 200 nm in order to ensure thereflectance thereof and the cooling rate.

[0572] The extinction coefficient k of the interference layer ispreferably 0.5 or less in order to cause light to be absorbed by therecording layer. The material of the interference layer may be selectedfrom ZnO, Ta₂O₅, SiO, Al₂O₃, Cu₂O, CuO, TaO, Y₂O₃, ZrO₂, CaF₂, MgF₂,Si₃N₄, AlN and mixtures thereof. In order to ensure the transmittance,the mean thickness of the interference layer is preferably 300 nm orless.

[0573] In addition, while the phase change optical disc has beendescribed in the above described examples, this preferred embodimentshould not be limited thereto, but it may be applied to any one ofvarious phase change recording media, such as an optical recording cardand an optical magnetic tape.

[0574] As described in detail above, according to this preferredembodiment, it is possible to provide a phase change recording mediumcapable of reproducing good address signals and servo signals, and it ispossible to provide a method and system for producing a phase changerecording medium, which can solve the problem that the initialcrystallizing step is complicated to deteriorate the productivity.

[0575] While the present invention has been disclosed in terms of thepreferred embodiment in order to facilitate better understandingthereof, it should be appreciated that the invention can be embodied invarious ways without departing from the principle of the invention.Therefore, the invention should be understood to include all possibleembodiments and modification to the shown embodiments which can beembodied without departing from the principle of the invention as setforth in the appended claims.

[0576] The entire disclosure of Japanese Patent Applications No.H10-181156 filed on Jun. 26, 1988 and No. H11-088010 filed on Mar. 30,1999 including specifications, claims, drawings and summaries areincorporated herein by reference in their entirely.

What is claimed is:
 1. A phase change recording medium comprising anas-deposited first recording layer configured to undergo a reversiblephase change between an amorphous state and a crystalline state due tolight irradiation and thereby change an optical characteristic, whereinsaid as-deposited first recording layer includes a plurality of finenuclei having an average size of 0.5 nm to 4 nm in said amorphous state.2. A phase change recording medium as set forth in claim 1, wherein saidas-deposited first recording layer is configured to form a crystal partincluding a plurality of crystal grains in the crystalline state whensaid as-deposited first recording layer is irradiated with an erasinglight beam, said plurality of crystal grains having grain sizes whosecrystal grain size distribution has at least two maxima.
 3. A phasechange recording medium as set forth in claim 2, wherein: said at leasttwo maxima has a first maximum for a grain size of greater than 4 nm andless than 20 nm and a second maximum for a grain size of greater than 20μm and less than 100 nm; and said crystal part comprises 75% or more ofall crystal grains in a first distribution centered on said firstmaximum and a second distribution centered on said second maximum.
 4. Aphase change recording medium as set forth in claim 1, furthercomprising: a second recording layer configured to undergo a reversiblephase change between an amorphous state and a crystalline state due tolight irradiation and thereby change a second recording layer opticalcharacteristic; and a separation layer provided between saidas-deposited first recording layer and said second recording layer.
 5. Aphase change recording medium as set forth in claim 1, wherein saidas-deposited first recording layer comprises at least one of Kr and Xein a range of between 0.25 at % to 10 at %.
 6. A phase change recordingmedium as set forth in claim 1, wherein said as-deposited firstrecording layer includes a continuous amorphous state band betweenadjacent tracks after said recording layer is irradiated with arecording light beam having a spot size of a e⁻² diameter greater than atrack pitch.
 7. A phase change recording medium as set forth in claim 2,wherein said as-deposited first recording layer includes a continuousamorphous state band between adjacent tracks after said recording layeris irradiated with a recording light beam having a spot size of a e⁻²diameter greater than a track pitch.
 8. A phase change recording mediumcomprising a first recording layer configured to undergo a reversiblephase change between an amorphous state and a crystalline state due tolight irradiation and thereby change an optical characteristic, whereinsaid first recording layer forms a crystal part including a plurality ofcrystal grains and having a crystal grain size distribution with atleast two maxima.
 9. A phase change recording medium as set forth inclaim 8, wherein: said at least two maxima has a first maximum for afirst grain size of greater than 4 nm and less than 20 nm and a secondmaximum for a second grain size of greater than 20 nm and less than 100nm; and said crystal part comprises 75% or more of all crystal grains ina first distribution centered on said first maximum and a seconddistribution centered on said second maximum.
 10. A phase changerecording medium as set forth in claim 8, wherein said first recordinglayer becomes the amorphous state when said first recording layer isirradiated with a recording light beam, and forms an amorphous partcomprising a plurality of fine nuclei having an average size of from 0.5nm to 4 nm.
 11. A phase change recording medium as set forth in claim 8,further comprising: a second recording layer configured to undergo areversible phase change between an amorphous state and a crystallinestate due to light irradiation and thereby change an opticalcharacteristic; and a separation layer provided between said firstrecording layer and said second recording layer.
 12. A phase changerecording medium comprising an as-deposited recording layer configuredto undergo a reversible phase change between an amorphous state and acrystalline state due to light irradiation and thereby change an opticalcharacteristic, said as-deposited recording layer having an amorphousstate thermal conductivity of from 0.8 W/mK to 6 W/mK.
 13. A phasechange recording medium as set forth in claim 12, wherein saidas-deposited recording layer comprises a plurality of fine nuclei havingan average size of from 0.5 nm to 4 nm when the as-deposited recordinglayer is in the amorphous state.
 14. A phase change recording medium asset forth in claim 12, wherein said as-deposited recording layercomprises: an address part substantially in the as-deposited recordinglayer in the amorphous state; and a data part.
 15. A phase changerecording medium as set forth in claim 12, wherein said as-depositedrecording layer comprises at least one of Kr and Xe in a range ofbetween 0.25 at % to 10 at %.
 16. A phase change recording mediumcomprising an as-deposited recording layer configured to undergo areversible phase change between an amorphous state and a crystallinestate due to light irradiation and thereby change a recording layeroptical characteristic, said as-deposited recording layer including acontinuous amorphous state band between adjacent tracks after saidas-deposited recording layer is irradiated with a recording light beamhaving a spot size of a e⁻² diameter greater than a track pitch.
 17. Aphase change recording medium as set forth in claim 16, wherein thecontinuous amorphous state band comprises a plurality of fine nucleiwith an average size of from 0.5 nm to 4 nm.
 18. A phase changerecording medium as set forth in claim 16, wherein said as-depositedrecording layer comprises at least one of Kr and Xe in a range ofbetween 0.25 at % to 10 at %.
 19. A method for producing a phase changerecording medium including a first recording layer part configured toundergo a reversible phase change between an amorphous state and acrystalline state due to light irradiation, and a second recording layerpart and configured to undergo a reversible phase change between anamorphous state and a crystalline state due to said light irradiation,comprising: depositing said first recording layer part on a firstsubstrate; forming a separation layer on said first recording layerpart; depositing said second recording layer part on a second substrate;irradiating said first recording layer part with an initialcrystallizing beam; irradiating said second recording layer partsubstantially simultaneously with the crystallizing step of said firstrecording layer part by using part of the initial crystallizing beampassing through said first recording layer; and sticking said first andsecond substrates together via said separation layer after saidcrystallizing steps so that said first recording layer part and saidsecond recording layer part oppose each other.
 20. A method forproducing a phase change recording medium having a substrate and anas-deposited recording layer including nuclei therein, comprising:raising a recording layer temperature to a temperature higher than aroom temperature using an infrared lamp; and maintaining a temperatureof said substrate less than a substrate thermal deformation temperatureduring the raising step by supporting said substrate on a materialconfigured to absorb substantially no light emitted from said infraredlamp during or after deposition of said recording layer on saidsubstrate.
 21. A method for producing a phase change recording medium asset forth in claim 20, wherein said material for supporting saidsubstrate is substantially transparent to the light emitted from theinfrared lamp.
 22. A phase change recording medium comprising anas-deposited first recording layer configured to undergo a reversiblephase change between an amorphous state and a crystalline state due tolight irradiation and thereby change a first recording layer opticalcharacteristic, the as-deposited first recording layer in thecrystalline state having a crystal grain size distribution having atleast two maxima and crystal grains of the at least two maxima randomlyarranged in the as-deposited first recording layer.
 23. A method forproducing a phase change recording medium including a first recordinglayer part configured to undergo a reversible phase change between anamorphous state and a crystalline state due to light irradiation, aseparation layer formed on said first recording layer part, and a secondrecording layer part formed on said separation layer and configured toundergo a reversible phase change between an amorphous state and acrystalline state due to said light irradiation, comprising: providing aconvergent optical system between said first and second recording layerparts to irradiate said second recording layer part with a condensedcrystallizing light beam, said convergent optical system beingconfigured to condense a light beam; irradiating said first recordinglayer part with an initial crystallizing light beam to crystallizeinitially; and irradiating said second recording layer part with part ofsaid initial crystallizing light beam passing through said firstrecording layer part and condensed by said convergent optical system tocrystallize initially substantially simultaneously with said firstrecording layer part.
 24. A phase change recording medium produced by aprocess comprising sputter depositing a recording layer onto a substratewith a relationship between a dc voltage (V_(dc)) applied to a targetand a sputter threshold voltage (V_(th)) of a target forming element setto be (V_(th))<(V_(dc))≦10(V_(th)).
 25. A phase change recording mediumproduced by a process as set forth in claim 24, further comprisinghaving an ion density (N_(i)) in a negative glow plasma produced in avicinity of said target during said sputter depositing in the range of(N_(i))>10¹¹.
 26. A phase change recording medium produced by a processas set forth in claim 24, wherein the recording layer includes Te.
 27. Aphase change recording medium produced by a process as set forth inclaim 24, wherein the recording layer includes Te and Sb.